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QOLUBL 


SCIENCE  FOR  THE  YOUNG; 

Oil, 

THE  FUNDAMENTAL  PRINCIPLES  OF  MODERN  PHILOSOPHY 
EXPLAINED  AND  ILLUSTRATED 

IN 

CONVERSATIONS  AND  EXPERIMENTS. 

AND    IN 

NARRATIVES  OF  TRAVEL  AND  ADVENTURE  BY  YOUNG 
PERSONS  IN  PURSUIT  OF  KNOWLEDGE. 


VOL.  II.— LIGHT. 


I/ 
SCIENCE  FOR  THE  YOUNG. 


LIGHT. 


BY  JACOB  ABBOTT, 


AUTHOR   OF 


'THE   FRANCONIA   STORIES,"    "MARCO  PAUL  SERIES,"    "YOUNG 

CHRISTIAN    SERIES,"    "HARPER'S    STORY   BOOKS," 

"ABBOTT'S  ILLUSTRATED  HISTORIES,"  &c 


WITH  NUMEROUS  ENGRA  VINGS. 


NEW    YORK: 

HARPER  &  BROTHERS,  PUBLISHERS, 

FRANKLIN     SQUARE. 


NOV  19  1900 


Entered  according  to  Act  of  Confess,  in  the  year  1871,  by 

HARPER   &   BROTHERS, 
In  the  Office  of  the  Librarian  ot  Congress,  at  NYasnington. 


Qc 

3t   I 

A/3 

OBJECT  OF  THE  WORK. 

THE  object  of  this  series,  though  it  has  been  prepared 
with  special  reference  to  the  young,  and  is  written  to  a 
considerable  extent  in  a  narrative  form,  is  not  mainly  to 
amuse  the  readers  with  the  interest  of  incident  and  ad- 
venture, nor  even  to  entertain  them  with  accounts  of  cu- 
rious or  wonderful  phenomena,  but  to  give  to  those  who, 
though  perhaps  still  young,  have  attained,  in  respect  to 
their  powers  of  observation  and  reflection,  to  a  certain 
degree  of  development,  some  substantial  and  thorough 
instruction  in  respect  to  the  fundamental  principles  of 
the  sciences  treated  of  in  the  several  volumes.  The  pleas- 
ure, therefore,  which  the  readers  of  these  pages  will  de- 
rive from  the  perusal  of  them,  so  far  as  the  object  which 
the  author  has  in  view  is  attained,  will  be  that  of  under- 
standing  principles  which  will  be  in  some  respects  new 
to  them,  and  which  it  will  often  require  careful  attention 
on  their  part  fully  to  comprehend,  and  of  perceiving  sub- 
sequently by  means  of  these  principles  the  import  and 
significance  of  phenomena  occurring  around  them  which 
had  before  been  mysterious  or  unmeaning. 

In  the  preparation  of  the  volumes  the  author  has  been 
greatly  indebted  to  the  works  of  recent  European,  and 
especially  French  writers,  both  for  the  clear  and  succinct 
expositions  they  have  given  of  the  results  of  modern  in- 
vestigations and  discoveries,  and  also  for  the  designs  and 
engravings  with  which  they  have  illustrated  them. 


CONTENTS. 


I.  RADIATION 13 

II.  WONDER   AND    MYSTERY 21 

III.  THE   VELOCITY   OF    LIGHT 30 

IV.  THE   LAW   OF   THE   SQUARES   OF   THE    DISTANCES 36 

V.  CANDLES   TOO   TALL , 44 

VI.  INTENSITY   OF   LIGHT. .r>4 

VII.  CANDLES   AND   LAMPS 60 

VIII.  THE   ARGAND   BURNER 71 

IX.  INTERMINGLING   OF   UNDULATIONS 75 

X.  REFLECTED   AND   TRANSMITTED    LIGHT 86 

XI.  SPECTRES    AND    GHOSTS 97 

XII.  THE   POLYTECHNIC    INSTITUTION 105 

XIII.   VERY    BRIGHT   LIGHTS 11G 

XIV.  COMBUSTION    OF   MAGNESIUM 124 

XV.  THE    MAGNESIUM    LAMP 131 

XVI.  INCANDESCENCE 130 

XVII.  FOLKESTONE 1 4 1 

XVIII.  THE    CHANNEL   AT   NIGHT 154 

XIX.  THE   ELECTRIC    LIGHT 163 

XX.  THE   CORRELATION   OF   FORCE 1 75 

XXI.  FRE8NEL 183 

XXII.  COLOR 194 

XXIII.  FLIPPY 205 

XXIV.  ILLUSIONS   EXPLAINED 215 

XXV.  FORMATION   OF   IMAGES 227 

XXVI.  LAWS    OF   REFLECTION    AND    REFRACTION 240 

XXVII.  THE   EYE 249 

XXVIII.  THE   RETURN 263 

XXIX.  FAREWELL   TO   FLIPPY 274 

XXX.   UP   THE   NORTH   RIVER 283 

XXXI.  LIGHTING    BY    GAS 295 

XXXII.  CONCLUSION SOU 

A  2 


ILLUSTRATIONS. 

Lawrence  and  John Frontispiece. 

The  Inverted  Image 17 

Illuminated  Vapors 25 

Illuminated  Spheres 26 

Velocity  of  Light— Astronomical  Determination 31 

Velocity  of  Light— Experimental  Determination 32 

Reading  the  Articles 41 

Enlargement  as  the  Squares  of  the  Distances 48 

Practical  Results 49 

Degree  of  Illumination 51 

Comparison  of  Shadows 55 

Law  Verified 55 

Comparison  of  Lights 56 

Ritchie's  Photometer 57 

Ancient  Lamp 65 

The  Police  of  old  Times 67 

The  Ducks  on  the  Lake 78 

Angles  of  Incidence  and  Reflection 87 

Mode  of  Measurement 88 

Apparent  Direction 91 

Reflection  from  Water 94 

The  Ghost  Illusion  on  the  Stage. 102 

Spectres  by  a  Double  Reflection 112 

The  Magic  Lantern 117 

Ancient  Light-house 118 

The  Magnesium  Lamp 132 

Water  from  Fire. 137 

Manufacture  of  Lamp-black 141 

The  Bat's  Wing 142 

Internal  Supply  of  Air 158 

Electrified  Points 165 

Charcoal  Points— magnified 168 

In  a  Vacuum ..; 170 


xii  ILLUSTRATIONS. 

Pag» 

Light-house  on  a  Rock .^ 1 77 

Magneto-electric  Machine.' ; . . .". . * .  k 1* 

Parabolic  Reflector lf 

Flashing  Light  by  Reflectors 186 

Convergence  of  Rays 1* 

Fresnel's  Idea 188 

Signal  Lantern 190 

Effect  of  the  Prisms 191 

Pole  seeming  to  be  bent 195 

Refraction  of  Solar  Ray ! 196 

Recomposition  of  Light 197 

Newton's  Disk 198 

Lines  of  the  Solar  Spectrum 199 

The  Spectroscope 200 

The  Gardens  of  the  Tuileries 208 

Masts  of  near  and  distant  Ships 217 

Near  and  distant  Birds 218 

Reflection  from  the  inner  Surface 222 

The  Plane  Mirror 229 

Seeing  through  a  Stone 234 

Image  reversed 236 

Enlargement  in  a  Concave  Mirror 237 

Distorted  Picture  reflected  true 238 

Diagram.     Law  of  Reflection 243 

Reflection  from  a  Concave  Surface 244 

Diagram.     Law  of  Refraction 245 

Image  of  the  Lily 251 

Section  of  the  Eye 252 

Camera  Obscura  in  a  Box 254 

Camera  Obscura  in  its  own  Building 255 

Camera  Obscura  in  a  Tent 257 

Photographic  Room 259 

The  Thaumatrope 2G1 

Atmospherical  Refraction 2G5 

Blowing  Bubbles 2G9 

Newton's  Bubble 271 

Rough  Plays 277 

Flippy  when  he  was  little 279 

Ancient  Lake  filled  up 291 

A  Wagon-load  of  Gas 303 

TL^  Locomotive OAQ 


/fl  2. 


CHAPTER  I. 

RADIATION. 

LIGHT  proceeding  from  a  luminous  object  tends  to  radi- 
ate in  all  directions.  If  the  luminous  object  is  a  candle, 
the  rays  can  only  diffuse  themselves  upward  and  on  the 
sides,  those  tending  downward  being  intercepted  by  the 
candlestick,  the  table,  and  the  ground. 

If  the  candle,  so  shining,  is  supposed  to  be  at  the  surface 
of  the  earth,  or  upon  any  horizontal  plane,  and  there  is 
nothing  to  intercept  its  rays  upward  or  on  any  side,  then 
it  is  plain  that  the  space  which  the  rays  illuminate  will  be 
of  the  form  of  a  hemisphere,  with  a  radius  equal  to  the  dis- 
tance through  the  air  to  which  the  light  could  penetrate. 
The  base  of  the  hemisphere  would  coincide  with  the  ground, 
or  the  horizontal  plane,  whatever  it  might  be,  on  which 
the  candle  was  placed,  while  the  spherical  surface  of  it 
would  extend  into  the  air,  forming  a  great  dome  over  and 
around  the  candle,  like  a  kind  of  lower  sky. 

Let  us  suppose  that  the  atmosphere  at  the  time  in  ques- 
tion is  so  clear  that  the  light  of  such  a  candle  would  be 
visible  for  a  distance  of  half  a  mile.  Then  the  radius  of 
the  hemisphere  in  the  atmosphere  which  would  be  illumin- 
ated— that  is,  the  distance  from  the  centre  to  the  outer 
boundary  of  it,  would  be  half  a  mile. 


14  RADIATION. 

But,  now,  if  we  suppose  that  instead  of  a  candle  at  the 
surface  of  the  earth  we  have  a  flame,  or  other  incandescent 
object,  of  the  same  size,  and  of  precisely  the  same  power 
to  radiate  light,  in  the  air  half  a  mile  above  the  surface 
of  the  ground,  then  the  hemisphere  that  was  illuminated 
would  become  a  sphere  or  globe,  the  diameter  of  which 
would  be  a  mile,  the  distance  from  the  centre  to  the  cir- 
cumference being  on  every  side  half  a  mile. 

The  light  of  this  candle,  or  of  the  luminous  object,  what- 
ever it  might  be,  so  placed,  would  be  barely  visible  to  any 
one  on  the  earth  looking  upward,  for,  by  the  supposition, 
half  a  mile  is  the  limit  of  the  distance  to  which  the  rays 
could  penetrate  through  the  atmosphere  and  retain  suffi- 
cient force  to  produce  their  proper  eifect  on  the  human 
eye.  An  eye  placed  any  where  else,  also,  on  the  margin  of 
the  illuminated  sphere,  and  directed  toward  the  centre, 
would  see  the  light.  So,  also,  if  an  eye  were  placed  any 
where  within  the  outer  boundary  of  the  sphere,  and  were 
directed  toward  the  centre  of  it,  it  would  see  the  light,  the 
impression  being  the  more  vivid  as  the  eye  making  the  ob- 
servation moved  in  from  the  outer  boundary  toward  the 
centre. 

We  must  not  suppose,  however,  that  such  an  illuminated 
sphere  as  we  have  described  would  have  any  precise  or 
definite  boundary.  Some  human  eyes  are  much  more  sen- 
sitive than  others,  and  can  see  a  much  fainter  light,  or,  in 
other  words,  can  see  a  given  luminous  object  at  a  much 
greater  distance  than  others.  The  eyes  of  some  animals, 
such  as  insects,  night  birds,  or  beasts  of  prey,  are  probably 
more  sensitive  than  any  human  eyes.  And  even  beyond 
the  limit  at  which  the  light  would  cease  to  produce  an  ef- 
fect upon  any  organ  of  vision,  some  of  its  radiations  may 
penetrate  and  produce  other  effects  of  which  we  have  no 
cognizance.  So  that  the  magnitude  of  the  sphere  which 


AN   ILLUMINATED   SPHERE.  15 

would  be  occupied  by  the  radiance  would  be  estimated 
very  differently  according  to  the  different  tests  of  the  pres- 
ence of  light  which  we  might  apply.  Still  the  portion  of 
space  that  the  radiation  would  fill  would  in  all  cases  be  a 
sphere,  with  the  luminous  source  itself  in  the  centre  of  it, 
since  the  limits  would  be  at  an  equal  distance  from  the 
centre  on  every  side,  whatever  might  be  the  test  by  which 
the  limits  were  determined. 

Thus  every  luminous  point,  the  radiation  from  which  is 
not  interrupted  on  any  side,  is  the  centre  of  an  illuminated 
sphere — illuminated  in  a  certain  sense,  as  will  be  presently 
explained — which  sphere  is  larger  or  smaller  according  to 
the  intensity  of  the  light  and  the  transparency  of  the  me- 
dium surrounding  it.  In  the  case  of  a  common  candle,  and 
in  an  ordinary  condition  of  the  atmosphere,  this  sphere 
might  perhaps  be  a  mile  in  diameter,  supposing  the  limits 
of  it  to  be  determined  by  the  powers  of  human  vision. 

The  sphere  thus  surrounding  the  luminous  point  is  filled 
with  light — that  is,  filled  in  a  certain  sense,  which  will  also, 
like  the  sense  in  which  it  is  illuminated,  be  presently  ex- 
plained. A  light  bright  enough  to  be  seen  at  a  distance 
oftfive  miles  would  in  the  same  manner,  if  its  radiance  were 
not  obstructed  in  any  direction,  form  the  centre  of  an  illu- 
minated sphere  ten  miles  in  diameter. 

The  sense  in  which  this  sphere  is  illuminated  or  filled 
with  light  is  this,  namely,  that  if  an  eye  is  placed  any 
where  within  it,  and  is  turned  toward  the  centre,  it  will  see 
the  light — that  is,  there  is  no  part  of  it  in  which  there 
would  be  found  any  space  as  large  as  the  pupil  of  the  hu- 
man eye,  and  probably  not  any  as  large  as  the  area  in- 
cluded by  the  eye  of  the  smallest  insect,  that  would  not 
furnish  rays  enough  to  form  an  image  upon  the  retina  so 
as  to  produce  vision. 

And  here  I  must  pause  a  moment  to  explain  how  it  is 


]  Q  RADIATION. 

that  an  image  is  formed  upon  the  retina  of  the  eye  so  as 
to  produce  vision. 

If  you  examine  one  of  the  glasses  of  a  pair  of  spectacles 
such  as  are  used  by  elderly  persons,  and  sometimes,  indeed, 
by  persons  who  are  still  young,  but  not  near-sighted,  you 
will  see  that  the  glass  is  thicker  in  the  middle  than  at  the 
edges.  Such  a  glass  is  called  a  convex  lens.  If,  now,  in 
the  evening,  you  remove  or  extinguish  all  the  lights  in  the 
room  but  one,  and  put  that  light  at  one  side,  or  in  one  cor- 
ner, and  then  proceed  to  the  opposite  corner,  or  into  the 
darkest  part  of  the  room,  and  there  hold  a  small  piece  of 
white  paper  against  the  wall,  and  one  of  the  glasses  of  the 
spectacles  between  it  and  the  light  at  the  proper  distance, 
you  will  find  that  an  image  of  the  candle,  inverted,  will  bo 
formed  upon  the  paper  or  card.  The  image  may  be  small, 
but  if  the  experiment  is  carefully  performed  it  will  be  beau- 
tifully distinct  and  clear.  The  lens  collects  and  concen- 
trates the  light,  and  forms  an  image  of  the  candle  upon  the 
paper  or  the  card,  which  serves  as  a  screen  to  receive  it. 
Of  course  this  experiment  can  be  performed  on  a  larger 
scale,  and  in  a  much  more  satisfactory  manner,  with  a 
proper  lens  and  other  convenient  apparatus,  as  shown  in 
the  following  engraving. 

Now  in  the  eye  there  is  just  such  a  lens  and  just  such  a 
screen — that  is,  just  such  in  respect  to  function.  The  lens 
is  in  the  front  part  of  the  eye,  and  the  screen,  which  is 
called  the  retina,  is  in  the  back  part ;  and  it  is  by  means 
of  this  image  on  the  retina  that  the  picture  of  the  outward 
object  is  conveyed  to  the  mind. 

Now  when  it  is  said  that  the  whole  of  the  illuminated 
sphere  surrounding  a  source  of  light  as  described  is,  in  a 
certain  seme,  filled  with  light,  the  meaning  is  that  there  is 
no  part  of  the  whole  space  where  an  opening  no  larger 
than  the  pupil  of  the  human  eye  will  not  take  in  enough  to 


EXPLANATION    OF  THE    EYE. 


17 


THE  INVESTED  IMAGE. 


form,  by  their  concentration  upon  the  retina,  an  image  of 
the  luminous  point  from  which  they  proceed,  just  as  a  lens, 
held  in  the  manner  I  have  described,  will  gather  rays 
enough  coming  from  the  candle  on  the  other  side  of  the 
room  to  form  an  image  of  the  candle  on  the  paper  screen. 

"Lawrence,"  said  John,  one  day,  as  he  had  been  reading 
about  this  in  a  book, "  here's  a  nice  experiment  for  me  to 
try,  if  I  only  had  a  pair  of  spectacles." 

"  Would  my  eye-glass  answer  the  purpose  ?"  asked  Law- 
rence. 

"  No,"  replied  John, "  I  think  not.  1  suppose  your  eye- 
glass is  concave,  and  what  I  require  is  a  convex  lens.  Let 
me  take  it  a  moment,  and  I  can  soon  tell." 

Lawrence  was  lying,  or,  rather,  reclining  on  a  sofa  in  the 
corner  of  the  room  near  a  window,  with  his  head  toward 
the  window,  so  that  the  light  fell  fair  upon  the  page  of  the 
book  which  he  was  reading.  John  was  sitting  at  a  table 
near.  Lawrence  unhooked  his  glass  from  the  cord  to  which 


jg  RADIATION. 

it  was  attached,  and  handed  it  to  John,  saying  at  the  same 

time,* 

"  There  it  is ;  but  find  out  whether  it  is  convex  or  con- 
cave with  your  eyes,  if  you  can,  and  not  with  your  fingers." 

"  Why  not  with  my  fingers  ?"  asked  John. 

"  You  can  feel  of  it  if  you  find  it  necessary,"  said  Law- 
rence, "but  the  less  we  touch  polished  glass  with  our  hands 
the  better.  There  are  always  particles  of  dust  floating  in 
the  air,  and  these  alight  on  our  fingers  and  on  the  glass, 
and  when  we  rub  our  fingers  over  the  glass  we  rub  the 
surface  with  these." 

"And  does  that  do  any  harm?"  asked  John. 

"  It  depends  upon  what  the  particles  of  dust  are  com- 
posed of,"  replied  Lawrence.  "  Some  of  them  are  minute 
fragments  of  cotton  or  woolen  fibres  worn  off"  from  clothes. 
They  would  not  do  much  harm.  Some  are  minute  spores 
of  plants." 

"  What  are  spores  ?"  asked  John. 

"  A  kind  of  seeds,"  said  Lawrence.  "  They  are  from  such 
plants  as  form  mould  and  mildew ;  and  some  smaller  still 
— so  small,  indeed,  that  the  plants  themselves  can  not  be 
seen  except  with  a  microscope ;  and  you  can  judge  how 
small  the  seeds  must  be.  These  would  not  do  much  harm 
any  more  than  the  woolen  and  cotton  abrasions.  But  there 
is  another  kind  of  dust  which  comes  from  the  road,  and 
which  consists  of  minute  scales  of  iron,  from  the  shoes  of 
the  horses,  and  the  tires  of  the  wheels,  or,  what  is  still 
worse,  of  fragments  of  stone  from  the  pavements,  some  of 
which  are  siliceous — that  is,  of  the  nature  of  flint,  and  are 
exceedingly  hard.  When  you  rub  these  over  the  glass 
with  your  fingers,  or  with  a  cloth,  or  a  piece  of  leather,  al- 
though no  one  rubbing  produces  any  perceptible  effect, 
after  a  time  the  fine  polish  begins  to  be  dimmed. 
*  See  Frontispiece. 


TAKING  CAKE  OF  LENSES.  19 

"  So  that,  if  you  are  going  to  study  optics,"  continued 
Lawrence, "  and  are  to  have  any  nice  lenses  and  prisms  to 
make  experiments  with,  I  advise  you  to  be  very  careful 
how  you  rub  them  with  dusty  fingers  or  dusty  cloths." 

"Yes, I  will,"  said  John. 

"  A  good  lens,"  said  Lawrence, "  is  a  -very  delicate  thing, 
and  sometimes  a  very  costly  thing.  It  requires,  in  the  first 
place,  a  very  nice  preparation  and  mixture  of  the  materials 
out  of  which  the  glass  is  made,  and  great  care  in  the  mak- 
ing of  it,  to  secure  its  being  uniform  and  homogeneous 
throughout,  so  as  to  act  upon  the  light  in  the  same  way  in 
every  part.  Then  it  is  a  very  nice  operation  to  grind  it 
precisely  to  the  true  form,  and  to  polish  it  perfectly.  So 
that,  when  you  get  a  good  lens,  if  you  ever  do  get  one,  you 
can't  be  too  careful  of  it." 

While  Lawrence  had  been  saying  these  things,  John  had 
been  attentively  examining  the  eye-glass,  without,  how- 
ever, touching  the  glass  at  all. 

"  Yes,"  said  he, "  yours  is  a  concave  lens ;  it  is  thinner 
in  the  middle  than  at  the  edges.  I  want  one  which  is 
thicker  in  the  middle  than  at  the  edges." 

"Perhaps  the  landlady  will  lend  you  her  spectacles," 
said  Lawrence. 

Lawrence  and  John  were  at  this  time  in  lodgings  in  Lon- 
don. The  keeper  of  the  lodging-house  where  they  had 
taken  their  rooms  was  quite  an  elderly  woman,  and  very 
soon  after  he  had  given  Lawrence's  eye-glass  back  to  him, 
John  heard  her  footsteps  in  his  bedroom,  which  was  a 
small  room  adjoining  their  sitting-room.  John  went  in 
immediately,  and  asked  her  if  she  used  spectacles.  She 
said  she  did  sometimes.  John  asked  her  if  she  was  willing 
to  lend  him  her  spectacles  a  few  minutes;  he  wished  to 
make  an  experiment  on  light  with  them. 

"  Certainly,"  she  said.     He  could  have  them  as  well  as 


20  RADIATION. 

not ;  but  they  were  a  poor  old  pair,  very  loose  in  the  joints, 
and  she  was  afraid  they  would  hardly  be  of  any  use  to 
him. 

John  said  that  the  looseness  of  the  joints  would  not  be 
of  any  consequence.  So  the  old  lady  went  out  and  pretty 
soon  returned  with  the  spectacles. 

John's  plan  was  to  go  into  his  own  room  to  try  his  ex- 
periment, as  it  would  be  necessary  to  darken  the  room,  he 
said,  and  he  did  not  wish  to  interrupt  his  cousin's  reading. 
But  Lawrence  said  he  would  like  to  see  the  experiment 
himself.  So  John  lighted  a  candle  and  closed  the  shutters, 
following  the  directions  given  in  his  book.  Then,  placing 
the  candle  on  one  corner  of  the  mantel-piece,  and  going  to 
the  farther  corner  of  the  room,  with  the  spectacles  and  one 
of  Lawrence's  cards  in  his  hand,  he  attempted  to  form  an 
image  on  the  paper  in  the  manner  we  have  already  de- 
scribed. 


JOHN'S    EXPERIMENT.  21 


CHAPTER  H 

WONDER   AND   MYSTERY. 

JOHN  was  quite  surprised  at  one  phenomenon  which  pre- 
sented itself  to  his  attention  in  performing  his  experiment. 
While  he  was  making  his  preparations,  Lawrence  remained 
on  the  sofa,  intending,  as  soon  as  he  found  that  John  had 
succeeded  in  bringing  the  light  to  a  focus  and  producing 
an  image  of  the  candle,  to  go  and  see  it.  But  John  seemed 
to  encounter  some  difficulty,  and  presently  he  said  that  he 
could  not  manage  the  spectacles,  for  he  could  not  keep  the 
bow  out  of  the  way. 

"  If  I  take  them  by  one  of  the  glasses,"  he  said,  "  and 
hold  the  other  glass  up  for  the  light  to  shine  through,  the 
upper  bow  falls  down  over  it,  the  joints  are  so  loose." 

"  Never  mind,"  replied  Lawrence ;  "  let  it  fall." 

"  Then  that  will  make  a  blank  bar  across  my  picture," 
said  John. 

"  No,"  replied  Lawrence ;  "  try  it  and  see." 

So  John  held  one  of  the  glasses  up,  with  the  bow  belong- 
ing on  that  side  hanging  down  over  it,  and  then,  placing 
his  card  against  the  wall,  he  moved  the  glass  to  and  fro, 
so  as  to  find  the  right  distance  for  producing  a  distinct 
image.  He  expected,  of  course,  that  the  shadow  of  the 
bow  would  be  seen  extending  from  above  down  over  the 
picture,  if  he  succeeded  in  producing  any  picture. 

But,  quite  to  his  surprise,  he  soon  obtained  a  very  excel- 
lent image  of  the  candle,  and  without  any  shadow,  or  dark 
bar,  or  any  other  indication  of  the  bow  at  all,  to  disfigure 
it.  The  image  was  reversed,  it  is  true — that  is,  it  was  up- 
side down — but  it  was  very  distinct  and  very  beautiful. 


22  WONDER  AND  MYSTEKT. 

There  was  the  flame  perfectly  formed,  though  pointing 
downward,  and  the  wick  (which  appeared  like  a  slender 
black  line  in  the  middle  of  it),  and  the  top  of  the  candle 
(which  was  rendered  bright  for  a  little  distance  by  the 
translucency  of  the  wax  at  the  margin),  all  plainly  to  be 
seen. 

John  was  very  much  pleased  to  find  his  experiment  so 
successful,  and  he  called  Lawrence  to  come  and  see  it. 
Lawrence  came,  and  he  showed  John  how  he  could  vary 
the  effect  by  changing  the  distance  of  the  lens  from  the 
candle,  though  this  made  it  necessary  also  to  change  the 
distance  from  the  lens  to  the  screen.  The  nearer  the  can- 
dle was  to  the  lens  on  one  side,  the  farther  it  was  necessary 
to  place  the  screen  on  the  other,  in  order  to  bring  the  rays 
to  a,  focus,  as  it  is  called— that  is,  to  make  the  image  dis- 
tinct. 

John  was,  however,  very  much  surprised  to  find  that 
there  was  no  dark  line  across  the  picture  of  the  candle 
corresponding  to  the  bar  formed  by  the  bow  of  the  spec- 
tacles. Lawrence  told  him  it  would  be  the  same  with  any 
opaque  substance  at  the  surface  of  the  glass.  He  might 
put  a  patch  directly  upon  the  glass  itself,  and  it  would  not 
show  as  a  spot  of  shadow  on  his  picture. 

John  tried  this  experiment.  He  cut  out  a  small  round 
piece  of  paper  about  as  large  as  the  section  of  a  pea,  and 
then,  wetting  it  to  make  it  adhere,  he  put  it  on  the  glass. 
Notwithstanding  what  Lawrence  had  told  him,  he  could 
not  help  expecting  to  see  it  produce  a  round  black  spot 
upon  the  image  of  the  candle.  But  it  did  not  do  so.  The 
image  became  somewhat  less  bright  than  before,  it  is  true, 
but  there  was  no  appearance  upon  it  of  any  shadow,  either 
from  the  bar  or  from  the  paper  patch. 

Lawrence  explained  to  him  how  this  was,  and  I  intend 
to  repeat  the  explanation  in  a  future  chapter ;  but  now  I 


DIVERGENCE    OP   KAYS.  23 

must  return  to  the  illumined  sphere,  which,  I  have  said, 
always  surrounds  every  luminous  point,  so  far  as  there  is 
no  object  intervening  to  intercept  the  rays. 

This  sphere,  as  has  already  been  said,  is  filled  with  light 
in  the  sense  that  in  no  part  of  it  can  an  eye  be  placed 
where  there  will  not  be  rays  enough,  if  the  eye  is  turned 
toward  the  luminous  point,  to  enter  the  pupil  and  form  an 
image  of  the  source  of  the  light  on  the  retina,  as  John 
formed  an  image  of  the  candle  on  his  card  by  the  lens. 
And  it  is  in  this  sense  only  that  the  space  within  the 
sphere  is  illumined,  namely,  that  it  is  completely  filled 
with  rays  of  light  proceeding  in  close  proximity  to  each 
other  from  the  centre  to  the  circumference. 

These  rays,  it  is  true,  diminish  in  intensity,  in  some  mys- 
terious way,  as  they  proceed  from  the  centre  to  the  circum- 
ference ;  but,  in  whatever  way  this  diminution  of  intensity 
is  eifected,  it  is  not  done  by  a  separation  of  the  rays  from 
each  other  as  they  diverge,  so  as  to  leave  some  parts  of  the 
space  empty. 

Radiation  from  a  luminous  point  is,  indeed,  often  in  books 
represented  by  lines  diverging  from  each  other  as  tliey  re- 
cede from  the  centre,  and  this,  however  closely  the  lines 
are  together  in  the  centre,  gives  us  the  idea  of  a  necessary 
separation  between  them  toward  the  outer  portions.  But 
we  must  not  imagine  that  the  diminution  in  the  intensity 
of  light,  as  the  distance  from  the  source  increases,  is  pro- 
duced by  any  separation  of  the  rays.  Exactly  how  we  are 
to  picture  this  diminution  of  intensity  to  our  minds  it  is 
difficult  to  say,  but  it  is  certain  that  it  is  not  the  result  of 
the  separation  of  lines  of  radiance  from  each  other  as  they 
recede,  leaving  intervals  between  them  dark. 

In  describing  the  phenomena  we  use  the  word  rays,  and 
we  represent  the  radiation  by  lines ;  but  we  must  conceive 
of  it,  so  far  as  we  can,  as  homogeneous  throughout,  and  as 


24  WONDER   AND   MYSTERY. 

diminishing  in  intensity,  when  it  does  so  diminish,  without 
the  least  interruption  of  continuity. 

The  sphere  thus  is  illuminated  only  in  this  sense,  that 
an  eye  in  any  part  of  it,  turned  toward  the  centre,  would 
see  the  light ;  looking  in  any  other  direction  through  the 
sphere,  it  would  see  nothing.  We  may,  however,  conceive 
such  a  sphere  to  be  illuminated  in  another  sense,  as  follows : 

If,  for  example,  the  spherical  space  were  filled  with  dust 
or  smoke,  or  any  other  substance  consisting  of  fine  parti- 
cles, and  if  the  light  in  the  centre  wrere  increased  in  intens- 
ity just  enough  to  make  up  for  the  loss  that  would  be  oc- 
casioned by  the  intercepting  of  the  light  by  sucli  particles, 
then  the  space  included  would  be  illuminated  in  another 
way.  The  sphere  itself  would  then  become  visible,  just  as 
the  sunbeams  do  when  shining  through  a  crevice  into  a 
dusty  or  smoky  room,  or  the  rays  of  the  sun  when  they  il- 
luminate the  mistiness  floating  among  the  clouds  at  even- 
ing in  the  western  sky,  and  which  people  call  "  drawing 
water,"  under  the  erroneous  idea  that  those  lines  of  light 
are  streams  of  vapor  ascending  into  the  air.  The  effect  is 
produced  by  the  rays  of  light  passing  through  the  inter- 
stices in  the  clouds,  and  then  shining  upon  and  being  re- 
flected by  the  particles  of  mist  which  they  meet  with  on 
the  way.  It  is  true  that  the  direction  of  the  illuminated 
lines  is  generally  downward,  as  there  is  usually  more  mist- 
iness in  the  atmosphere  near  the  earth  than  above,  though 
they  are  sometimes  seen  ascending  as  well  as  descending, 
as  is  represented  in  the  following  engraving. 

If  now  the  sphere  surrounding  the  luminous  point  which 
I  have  been  describing  were  illuminated  in  this  manner — 
that  is,  by  having  particles  floating  in  the  air  to  receive 
and  reflect  portions  of  the  light — with  a  sufficiently  in- 
creased intensity  at  the  source  to  just  make  up  for  the 
loss,  the  form  and  the  extent  of  it — that  is,  of  the  whole 


UNSEEN   LIGHT. 


25 


ILLUMINATED   VAPOE8. 


sphere— would  be  visible  to  the  eye,  appearing  like  a  vast 
ball  of  light  a  mile  in  diameter,  bright  at  the  centre,  and 
gradually  diminishing  in  brightness  from  the  centre  to  the 
outer  surface,  where  the  light,  by  insensible  gradations, 
without  any  definite  boundary,  would  melt  into  the  dark- 
ness and  disappear. 

It  must  be  understood,  however,  that  this  sphere  would 
be  thus  visible  to  us,  not  by  means  of  any  of  the  rays  of 
light  which  were  passing  out  from  the  centre  to  the  cir- 
cumference of  the  sphere  on  their  regular  course,  but  only 
by  means  of  that  portion  of  them  which  was  intercepted 
on  the  way  and  reflected  to  the  eye  by  the  solid  particles. 

And  this  brings  to  our  minds  a  principle  of  fundamental 
importance,  namely,  that  no  light  produces  any  effect  upon 
our  vision  except  such  as  passes  into  the  eye.  It  may  pass 
before  us  or  across  our  field  of  view  in  any  quantity  and 
of  any  intensity  without  being  perceptible  to  us  at  all. 
It  is  only  when  it  enters  the  eye,  and  falls  upon  the  screen 
called  the  retina,  in  the  back  part  of  it,  that  we  can  have 
any  consciousness  of  its  presence. 

Thus,  if  such  a  candle  as  we  have  supposed  were  sur- 
B 


26  WONDER   AXD   3IYSTEKY. 

rounded  by  an  atmosphere  so  far  transparent  as  not  to 
contain  any  substances  capable  of  sensibly  reflecting  the 
light,  the  light  would  go  on  diminishing  in  intensity  as  it 
receded  from  the  centre  until  it  disappeared,  and  in  that 
sense  the  whole  sphere  would  be  illuminated — that  is,  it 
would  befitted  with  a  radiation  of  light;  but  in  looking 
toward  it  we  should  see  nothing  except  the  luminous  point 
in  the  centre,  and  we  should  not  see  that  unless  our  eyes 
were  turned  directly  toward  it. 

And  now  let  us  suppose  that,  instead  of  one  luminous 
point  or  candle  flame,  there  were  two,  and  that  they  were 
placed  at  the  distance  of  half  a  mile  from  each  other;  the 
two  illuminated  spheres  would  then  interpenetrate  each 
other,  so  to  speak,  to  the  extent  of  half  their  diameters. 
The  rays  from  A,  proceeding  in  the  direction  toward  B, 
would  encounter  those  of  B  coming  toward  A.  The  en- 
counter would  be  direct  on  the  line  joining  the  two  central 
points,  and  in  all  other  parts  it  would  be  indirect,  and  the 
crossing  would  be  at  various  angles. 


LT.CMINATEB  SPHEEE8. 


In  those  parts  of  the  space  common  to  both  spheres 
which  are  equidistant  from  the  two  centres,  the  two  ra- 
diances encountering  each  other  would  be  equal.  In  those 
parts  which  were  nearer  to  one  than  to  the  other,  the  light 


EMANATIONS    OF   LIGHT.  27 

coming  from  the  nearest  point  would  be  strongest.  Thus 
the  radiances  issuing  from  the  two  centres  would  encounter 
each  other,  in  the  space  common  to  both,  at  every  possible 
angle  and  with  every  possible  disparity  of  force. 

And  yet,  so  far  as  we  can  discover,  such  rays  do  not  in- 
terfere with  each  other  in  the  least ;  for,  wherever  you  put 
your  eye  within  the  portion  traversed  by  the  light  from 
both  the  centres,  if  you  turn  your  eye  to  either,  you  have 
its  image  as  clearly  and  distinctly  painted  on  your  retina 
as  if  the  other  did  not  exist ;  that  is  to  say,  the  radiance 
which  comes  from  one  of  the  points,  though  its  track  has 
been  crossed  all  the  way  by  the  emanation  from  the  other 
— both  filling  the  space  completely — is  not  disturbed  or  in- 
terfered with  by  the  other  in  the  slightest  degree. 

This  is  wonderful,  and  it  is  for  the  purpose  of  furnishing 
a  clear,  simple,  and  precise  idea  of  the  nature  of  this  mys- 
tery, as  the  foundation  of  a  right  understanding  of  what 
is  to  follow,  that  I  have  made  these  suppositions  of  candles 
in  the  air. 

If  you  have  followed  what  I  have  said  closely  enough  to 
have  received  distinctly  an  idea  of  the  nature  of  this  won- 
der of  the  non-interference,  in  the  ordinary  sense,  of  lumin- 
ous emanations  moving  in  contrary  directions,  and  cross- 
ing each  other  at  every  possible  angle  and  on  the  same 
track,  you  are  prepared  to  appreciate  in  some  degree  the 
amazing  magnitude  and  extent  of  it.  Every  star  in  the 
sky  is  the  centre  of  a  sphere  that  is  illuminated  by  its  ra- 
diation in  the  manner  I  have  explained — a  sphere,  too, 
which  is  so  enormous  in  extent  that  its  magnitude  and 
grandeur  surpass  all  human  conception. 

Light  is  proved  to  move  at  the  rate  of  between  one  and 
two  hundred  thousand  miles  in  a  second,  which  is  sufficient 
to  carry  it  round  the  earth  in  the  seventh  part  of  a  second, 
and  there  are  stars  so  remote  that  it  would  require  Hun- 


28  WONDER   AND   MYSTERY. 

dreds  of  years  for  their  light,  moving  at  that  inconceivably 
great  velocity,  to  reach  the  earth !  Think  of  the  enormous 
magnitude  of  a  sphere  traversed  by  the  radiance  of  such  a 
star !  Now  every  one  of  the  millions  of  stars — and  if  we 
include,  as  we  ought  to  do,  with  those  that  can  be  seen  by 
the  naked  eye,  those  which  are  brought  to  view  by  the  tel 
escope,  either  as  single  stars  or  are  resolved  from  nebulae, 
the  number  is  to  be  reckoned  at  thousands  of  millions — is 
surrounded  by  a  sphere  of  radiance  which  must  extend  to 
us,  and  all  these  spheres  occupy  the  same  space,  entering 
into,  crossing,  and  interpenetrating  each  other  in  every  di- 
rection, so  that,  when  you  hold  up  a  needle  in  the  evening 
air  on  a  clear  night,  there  are  millions  upon  millions  of  dis- 
tinct radiations  passing  through  the  eye  of  it  in  both  direc- 
tions, encountering  each  other  at  every  possible  angle  and 
with  every  conceivable  disparity  of  force.  And  yet  each 
one  of  these  radiations  maintains  its  way  so  entirely  unin- 
terrupted and  undisturbed  by  the  rest,  that  you  can  select 
any  one  of  them  you  choose,  and,  by  conducting  a  suffi- 
cient portion  of  it  from  the  space  around,  by  means  of  the 
telescope,  to  your  eye,  you  can  there  produce  a  picture  of 
its  source  upon  the  retina  as  clear,  and  distinct,  and  as 
sharply  defined  as  if  its  own  radiance  was  the  only  one 
emitted,  and  had  the  entire  and  exclusive  occupation  of 
the  field. 

There  is  no  way  of  escaping  from  or  diminishing  the  un- 
utterable wonderfulness  of  these  facts.  I  have  only  called 
the  emission  a  radiance  —  that  is,  something  radiated  — 
without  intending  to  say  in  what  it  consists.  It  has  been 
thought  to  consist  of  streams  of  infinitely  minute  particles 
of  matter.  It  is  now  generally  considered  as  an  undula- 
tion or  vibration  in  some  extremely  subtle  medium  dif- 
fused through  space,  to  which  the  name  luminiferous  ether 
has  been  given,  which  phrase  means,  simply,  the  unknown 


SYMBOLS   FOB   DESIGNATING   LIGHT.  29 

something  which  transmits  the  light.  It  is  supposed  that 
one  or  the  other  of  these  two  suppositions  must  be  correct, 
because  these  are  the  only  two  ways  in  which  we  can  con- 
ceive that  action  of  any  kind  can  be  conveyed  through 
space.  But  whether  there  may  not  be  modes  of  transmis- 
sion for  force  that  man,  with  his  present  mental  constitu- 
tion, can  not  conceive  of,  is  a  grave  question. 

At  any  rate,  it  is  now  universally  agreed  among  scien- 
tific men  to  regard  light  as  transmitted  by  a  series  of  un- 
dulatory  movements  in  an  intervening  medium,  and  all 
the  calculations  and  all  the  language  used  in  describing 
the  phenomena  are  based  at  the  present  day  on  this  hy- 
pothesis. In  all  the  drawings,  however,  and  other  illustra- 
tions intended  to  represent  the  action  of  light  to  the  eye, 
the  radiation  is  represented  by  lines,  which  are  more  ap- 
propriate to  the  idea  of  a  progressive  motion  of  streams 
of  particles  than  to  that  of  undulations.  We  are  obliged 
to  use  both  these  modes,  as  the  best  symbols  of  thought 
at  our  command ;  but  when  we  attempt  to  pass  from  these 
symbols  to  the  realities  which  they  are  intended  to  repre- 
sent, we  are  lost  in  wondering  what  the  actual  nature  of 
emanations  can  be  that  can  thus  meet,  and  cross,  and  en- 
counter each  other  in  every  imaginable  way — in  such 
countless  numbers,  within  such  inconceivably  narrow  lim- 
its, and  at  such  inconceivably  rapid  rates  of  motion — and 
each  of  the  millions  of  separate  motions  pursue  its  own 
way  without  being  in  the  least  degree  deranged,  disturbed, 
or  interfered  with  by  the  rest.  We  ask  ourselves  in  amaze- 
ment, What  can  the  emanation  be,  in  its  intrinsic  nature, 
that  can  exist  under  such  conditions  as  these?  What  is 
light  ?  We  can  not  tell.  We  can  really  know  nothing  of 
its  essential  nature.  We  can  only  study  such  of  its  modes 
of  action  and  such  of  its  effects  as  come  within  the  reach 
of  our  senses  and  of  our  half-developed  reasoning  powers. 


30  THE   VELOCITY    OF   LIGHT. 


CHAPTER  III. 

THE    VELOCITY    OF   LIGHT. 

IT  may  seem  Btrange,  as,  indeed,  it  really  is,  that,  since 
light  moves  at  such  a  velocity  as  to  carry  it  seven  times 
around  the  earth  in  a  second,  there  can  be  any  possible 
way  by  which  its  velocity  can  be  measured.  But  many 
ways  of  doing  this  have  been  discovered  or  devised. 

Some  of  these  methods  are  astronomical — that  is,  the 
velocity  of  light  is  determined  by  observations  of  certain 
movements  and  appearances  among  the  heavenly  bodies, 
and  by  computations  made  from  them.  Fully  to  under- 
stand these  computations,  and  the  astronomical  principles 
on  which  they  are  based,  requires  a  degree  of  mathematical 
and  astronomical  knowledge  which  few  persons  have  time 
to  acquire.  But  astronomers  have  given  such  abundant 
proof  of  the  soundness  and  trustworthiness  of  their  meth- 
ods, in  the  exactness — to  a  second — with  which  their  pre- 
dictions in  respect  to  eclipses,  transits,  occultations,  and 
other  celestial  phenomena  are  always  fulfilled,  that  when 
they  agree  in  assuring  us  that  they  have  determined  any 
point  connected  with  celestial  phenomena,  we  have  every 
possible  reason  for  placing  confidence  in  the  result. 

A  general  idea,  moreover,  of  one  of  the  methods  adopt- 
ed can  be  obtained  by  the  aid  of  the  following  engraving. 
The  method  consists  in  making  first  an  exact  computation 
of  the  time  when  some  astronomical  phenomenon  will  actu- 
ally occur,  and  then  observing  the  difference  in  the  time 
in  which  it  is  seen  to  occur  by  an  observer  on  the  earth 
when  the  earth  is  on  opposite  sides  of  its  orbit.  The  phe- 


ECLIPSE  OF  JUPITER'S  SATELLITE.  31 

nomenon  most  convenient  for  such  purposes  as  this  is  an 
eclipse  of  some  one  of  the  satellites  of  Jupiter. 


VELOCITY   OF   LIGHT — ASTRONOMICAL   DETERMINATION. 

In  the  engraving,  e  represents  the  satellite  about  enter- 
ing the  shadow  of  the  planet.  The  precise  moment  at 
which  it  really  enters  is  known  beforehand,  and  it  is  found 
by  accurate  observation  that  the  apparent  time  of  its  en- 
tering, as  seen  from  the  earth  when  it  is  at  T,  in  that  part 
of  its  orbit  which  is  nearest  the  planet,  is  a  certain  number 
of  minutes  sooner  than  when  it  is  observed  when  the  earth 
is  at  t — that  is,  in  the  part  of  its  orbit  which  is  most  re- 
mote. From  these  data,  the  time  required  for  the  light  to 
pass  through  the  diameter  of  the  earth's  orbit  is  deter- 
mined. 

The  conclusions  deduced  from  astronomical  observations 
like  these  have  been  abundantly  confirmed  by  ingenious 
devices  which  have  been  contrived  for  measuring  the  ve- 
locity of  light  on  the  earth's  surface.  The  engraving  on 
the  following  page  represents  one  of  these  methods,  the 
principle  of  which,  with  a  little  attention,  can  be  easily  un- 
derstood, though  it  would  require  a  great  deal  of  practical 
experience  and  skill,  and  very  delicate  powers  of  observa- 
tion, to  enable  any  person  to  perform  the  experiment  suc- 
cessfully with  it,  so  as  to  arrive  at  a  satisfactory  result. 

You  will  see  by  the  engraving  that  the  apparatus  con- 
sists of  two  separate  parts,  which  are  connected  by  dotted 


32  THE    VELOCITY    OF   LIGHT. 


TELOCITY   OF  LIG11T—  EXPERIMENTAL    DETERMINATION. 

lines.  In  using  it,  the  two  parts  are  placed  at  the  distance 
of  several  miles  from  each  other,  the  separation  being  rep- 
resented by  the  break  in  the  dotted  lines.  The  left-hand 
part  of  the  apparatus  is  simply  a  hollow  tube,  having  at 
the  right-hand  end  of  it  a  lens,  and  at  the  other  end  a  mir- 
ror, which  are  so  adjusted  that  a  beam  of  light  entering 
through  the  lens  shall  be  brought  to  a  focus  on  the  mirror, 
and  then  reflected  back  through  the  lens  again  on  the  same 
path  by  which  it  came  in. 

That  an  incoming  and  an  outgoing  radiation  can  thus 
pass  through  the  same  tube,  in  contrary  directions,  at  the 
same  time,  without  in  the  least  degree  interfering  with  or 
deranging  each  other,  is  only  another  example  of  the  won- 
derful action  of  this  mysterious  power  that  was  described 
in  the  last  chapter. 

The  portion  of  the  apparatus  toward  the  right  is  set  at 
the  place  where  the  observation  is  to  be  made,  the  other 
part,  as  has  already  been  said,  being  placed  at  as  great  a 
distance  as  possible,  but  within  view.  This  second  part  of 
the  apparatus,  like  the  other,  consists  of  a  tube,  with  a 
branch  near  one  end  of  it,  which  is  open  toward  the  source 


APPARATUS   FOE   TIMING   LIGHT.  38 

of  light.  As  the  light  enters  the  tube,  the  rays,  of  course, 
are  diverging.  Near  the  entrance  they  pass  through  a 
lens  by  which  they  are  made  parallel.  A  little  farther  on 
they  pass  through  another  lens,  by  which,  from,  being  par- 
allel, they  are  made  to  converge,  but  before  coming  to  a 
focus  they  strike  the  plate  of  glass,  M.  This  glass,  though 
not  strictly  a  mirror — not  being  silvered  on  the  back — re- 
flects a  large  portion  of  the  rays,  turning  them  into  the  main 
tube,  but  without,  however,  changing  their  convergency. 

On  the  other  side  of  the  tube  is  a  system  of  clock-work 
moved  by  a  weight  attached  to  a  cord  that  is  wound  round 
a  drum  seen  at  the  end.  Those  who  are  interested  in 
tracing  out  the  connections  of  machinery  will  see  that 
there  are  four  axles  in  this  work.  The  first  is  the  axle  of 
the  drum.  Near  the  end  of  this  axle,  toward  the  right,  is 
a  large  toothed  icheel,  which  carries  a  small  wheel  upon  tho 
end  of  the  next  axle.  On  the  left-hand  end  of  this  second 
axle  is  another  large  wheel,  which  carries  a  small  wheel  on 
the  third  axle.  This  same  system  of  large  wheels  carrying 
small  ones  is  carried  through  the  fourth  and  fifth  axles, 
and  thus  a  very  great  velocity  is  imparted  to  the  fifth  by 
the  descent  of  the  weight. 

On  the  left-hand  end  of  the  fifth  axle  is  a  large  wheel, 
which  you  see  enters  into  the  tube  through  a  slit  made  in 
the  side  of  the  tube  for  admitting  it.  The  margin,  or  cir- 
cumference of  this  wheel,  is  cut  into  alternate  notches  and 
teeth,  square  in  form  and  equal  to  each  other,  and  the  wheel 
is  so  adjusted  in  respect  to  the  tube  at  the  point  where  the 
beam  of  light  passing  through  is  just  coming  to  a  focus, 
that  each  tooth,  as  it  moves  by,  shall  stop  a  beam  of  light, 
while  the  notch  that  follows  shall  allow  it  to  pass.  Thus, 
when  the  wheel  is  rapidly  revolving,  a  succession  of  flashes 
will  pass  out  through  the  tube,  following  each  other  with 
inconceivable  rapidity. 

R  ? 


34  THE   VELOCITY   OF   LIGHT. 

Let  us  now  suppose  for  a  moment  that  the  weight  is  dis- 
connected from  the  drum,  but  that  there  is  a  handle  at- 
tached to  the  axis  of  it,  by  means  of  which  we  can  turn  the 
wheel  at  any  rate  of  velocity  we  choose.  Let  us  also  sup- 
pose that  the  other  part  of  the  instrument — that  is,  the 
part  that  we  placed  on  a  distant  hill,  is  so  far  away  that 
it  would  require  one  second  for  the  light  to  pass  over  the 
adjoining  country  to  it,  enter  the  tube,  be  reflected  to  the 
end  of  it,  and  return.  This  would  be  impossible  in  fact, 
since  light  travels  at  a  rate  which  would  carry  it  seven 
times  round  the  earth  in  that  time.  We  can,  however,  sup- 
pose it  for  the  purpose  of  illustrating  the  action  of  the  ap- 
paratus. Let  us  suppose  that  we  allow  a  flash  to  pass  out 
through  a  notch,  and  that  an  observer  with  his  eye  at  A  is 
watching  for  its  return.  At  the  expiration  of  the  second, 
the  time  required  for  its  journey,  he  would  see  it  through 
the  glass,  M,  which,  it  will  be  remembered,  was  not  silver- 
ed, but  only  polished,  so  that,  while  it  reflected  a  portion 
of  the  light,  it  also  allowed  a  portion  to  pass  through. 

But  if,  on  the  other  hand,  the  wheel  were  to  be  moved 
while  the  ray  of  light  was  gone,  so  that,  on  the  return,  it 
should  find  a  tooth  in  its  way  to  stop  it,  instead  of  an  open- 
ing to  allow  it  to  pass  through,  it  is  plain  that  the  «ye  at 
A  would  see  nothing.  And,  moreover,  if  the  wheel  were 
made  to  revolve  regularly  at  a  rate  which  should  bring  a 
notch  and  a  tooth  alternately  into  the  path  of  the  ray  at 
intervals  of  a  second,  then  every  flash  which  went  out 
through  the  notch  would  find  a  tooth  in  its  way  to  intei-- 
cept  it  when  it  came  in,  and  the  eye  at  A  would  not  see 
the  light  at  all. 

If  it  required  less  than  a  second,  as,  indeed,  it  actually 
must,  for  the  ray  of  light  to  pass  to  and  fro,  then  all  that 
would  be  required  to  stop  the  flashes  on  their  return  would 
be  to  make  the  wheel  turn  faster;  and  it  is  easy  to  see 


RESULT    OF   THE    EXPERIMENT.  35 

that,  from  the  degree  of  speed  which  it  would  be  found 
necessary  to  give  to  the  wheel  in  order  to  bring  the  teeth 
up  rapidly  enough  into  the  path  to  stop  every  flash  on  its 
return,  it  would  be  easy  to  determine  the  time  required 
for  making  the  journey.  You  will  see,  by  a  careful  inspec- 
tion of  the  figure,  that  there  are  two  little  indexes  at  the 
ends  of  the  fourth  and  fifth  axles,  by  which  the  speed  of 
the  wheels  in  this  instrument  is  registered,  so  that  the  com- 
putation can  easily  be  made.  The  result  of  a  trial  made 
with  the  apparatus  near  Paris  corresponded  very  nearly, 
in  respect  to  the  velocity  of  light,  with  those  which  had 
been  obtained  by  the  astronomical  calculations. 

I  have  described  this  contrivance  in  detail,  both  because 
it  is  very  useful  to  learn  to  understand  tho  nature  and  ac- 
tion of  mechanism  from  engravings  and  descriptions,  and 
also  because  this  case  is  a  striking  instance  of  the  ingenu- 
ity and  skill  which  have  been  exercised  by  scientific  men 
in  discovering  secrets  of  nature  which  we  might  have 
thought  it  hopeless  to  attempt  to  unfold.  The  idea  of  at- 
tempting to  find  any  means  of  actually  measuring,  with- 
in a  space  of  a  few  miles,  the  velocity  of  a  motion  swift 
enough  to  pass  seven  times  round  the  earth  in  a  second, 
would  have  seemed  to  every  one,  at  first  view,  to  be  utter- 
ly chimerical. 

Do  not  forget  the  result,  which  is,  that  the  velocity  with 
which  light  moves  is  such  as  to  carry  it  about  175,000 
miles,  or  seven  times  round  the  earth,  in  a  second.  The 
rays  require  about  eight  minutes  to  come  to  us  from  the 
sun. 


36        THE   LAW   OF   THE    SQUARES   OF   THE    DISTANCES. 


CHAPTER  IV. 

THE   LAW    OF   THE   SQUARES    OF   THE    DISTANCES. 

I  MUST  admit  that  the  title  of  this  chapter  is  not,  at  the 
first  view,  at  all  an  attractive  one.  It  sounds  very  mathe- 
matical. But  then  there  is  an  interest  and  a  beauty  in  a 
mathematical  principle  when  it  is  once  understood,  and  as 
this  one,  known  as  the  Law  of  the  Squares  of  the  Distances, 
can  "be  easily  understood  when  properly  explained,  and  as 
it  is  one  of  fundamental  importance,  not  only  in  its  appli- 
cation to  the  subject  of  light,  but  in  countless  other  cases 
where  we  may  observe  its  operations  in  the  phenomena 
of  nature,  I  hope  that  none  of  the  more  intelligent  and 
thoughtful  of  the  readers  of  this  book  will  be  alarmed  at 
the  mathematical  aspect  of  its  name. 

The  circumstances  under  which  John's  attention  was 
first  called  to  it  were  somewhat  curious.  He  and  his 
cousin  Lawrence  had  been  making  an  excursion  that  day 
to  the  Tower  of  London,  a  famous  old  structure,  which  was 
used  in  former  times  as  a  fortress  to  defend  the  city  from 
hostile  vessels  coming  up  the  river.  Of  course,  since  this 
was  its  object,  it  was  below  the  city  at  the  time  when  it 
was  built,  but  the  city  has  now  extended  far  below  the 
spot  on  which  it  stands.  I:  is,  for  other  reasons  also,  now 
useless  for  any  purposes  of  defense,  but  it  is  still  preserved, 
and  is  used  as  a  museum  of  curiosities,  and  contains  vast 
collections  of  ancient  arms  and  armor,  and  of  a  great  many 
other  relics  of  old  times  which  are  very  curious  to  see. 

Lawrence  and  John  had  been  to  visit  it  that  day,  and 
had  stopped  on  their  return  to  their  lodgings  to  dine  at  a 
coffee-house ;  for,  as  it  was  uncertain  what  time  they  would 


JOHN'S  TOUR  ABROAD.  37 

return,  they  had  concluded  not  to  make  arrangements  for 
having  dinner  at  home.  It  was  eight  o'clock  when  they 
arrived,  and,  when  they  went  into  their  sitting-room,  the 
housemaid  went  before  them  and  lighted  the  candles  which 
stood  on  the  table  in  the  middle  of  the  room.  They  were 
two  very  tall  candles,  in  two  very  tall  candlesticks,  so  that 
the  flames  of  the  candles  were  about  two  feet  above  the 
table. 

John  went  to  the  sofa  and  sat  down  upon  it,  as  if  he 
were  glad  to  find  a  place  where  he  could  rest. 

"  I'm  tired,"  said  he,  "  and  yet  I've  an  hour's  work  to  do 
before  I  go  to  bed." 

"  How  is  that  ?"  asked  Lawrence. 

"  Why,  I  have  a  half  hour  more  of  study  to  do,  and  then 
it  will  take  me  full  half  an  hour  to  write  about  our  visit  to 
the  Tower  in  my  journal.  I  must  not  let  my  journal  get 
behindhand." 

In  order  to  explain  John's  remark  that  he  had  half  an 
hour  more  to  study  that  evening,  I  must  relate  how  it  was 
that  he  came  to  make  this  voyage  to  Europe  with  his 
cousin.  His  cousin  had  just  graduated  at  the  scientific 
school,  and  had  formed  a  plan  to  go  and  spend  some 
months  in  Paris,  in  order  to  pursue  still  farther  certain 
branches  of  science  for  which  there  were  great  facilities  in 
that  city,  and  also  to  visit  and  examine  certain  great  engi- 
neering works  which  had  been  constructed  in  England  and 
France.  He  was,  in  fact,  educating  himself  to  be  an  engi- 
neer. 

John,  when  he  heard  of  his  cousin's  design,  felt  a  strong 
desire  to  accompany  him.  He  proposed  the  plan  to  his 
mother.  She  was  at  first  somewhat  surprised  at  the  prop- 
osition, but,  the  more  she  thought  of  it,  the  more  she  was 
pleased  with  the  idea.  She  said  that  she  would  speak  to 
his  father  about  it. 


38        THE   LAW   OF   THE    SQUARES    OF   THE   DISTANCES. 

When  she  proposed  the  plan  to  her  husband  the  next 
morning  at  breakfast,  he  at  first  shook  his  head  somewhat 
doubtfully,  saying,  "  It  Avill  make  a  great  interruption  in 
his  studies." 

"  It  will  help  him  very  much  in  his  French,  at  least," 
said  John's  mother. 

"True,"  said  his  father;  "it  will  help  him  decidedly  in 
his  French." 

"  And  it  will  be  a  great  advantage  to  him  every  way," 
said  his  mother,  "  to  see  a  little  of  the  world." 

"And  then, besides,"  said  John,  "I  can  go  on  with  my 
studies  in  other  things.  Cousin  Lawrence  is  an  excellent 
teacher." 

"Can  you  study  while  you  are  traveling?"  asked  his 
father. 

"Yes,  sir,"  said  John, promptly.  "I  can  have  my  book 
and  study  my  lesson  in  the  cars  just  as  well  as  any  where 
else.  That  would  not  prevent  my  looking  out  of  the  win- 
dow now  and  then." 

"Well,"  said  his  father,  after  a  moment's  pause,  "I'll  see. 
Talk  with  your  cousin  about  it,  and  see  what  he  says. 
Form  a  definite  plan,  and  show  it  to  me,  and  I  will  con- 
sider it." 

So  John  went  that  same  day  to  find  his  cousin,  and 
brought  the  question  before  him.  His  cousin  seemed  very 
much  pleased  with  the  idea  of  having  John  for  his  compan- 
ion, and  said  that  he  would  draw  up  some  kind  of  a  plan 
in  the  form  of  conditions,  and  that  then,  if  John  agreed  to 
them,  he  could  offer  them  to  his  father. 

A  few  days  after  this,  Lawrence  presented  a  paper  to 
John  containing  the  conditions,  asking  him  to  examine 
them  and  see  if  he  was  willing  to  agree  to  them,  or 
whether  he  would  wish  to  have  any  alterations  made. 
John  examined  the  conditions  attentively.  There  was 


THE    CONTRACT   WITH    LAWRENCE.  39 

only  one  alteration  that  he  suggested,  and  that  was,  that 
in  the  article  which  specified  that  he  was  to  study  so  much 
every  day,  the  words  Sundays  excepted  should  be  inserted. 
Lawrence  said  that  was  meant  to  be  understood,  but  that 
it  would  be  better  to  have  it  expressed.  So  the  words 
were  put  in.  Some  other  minor  changes  were  also  made. 

"And  now,"  said  Lawrence,  when  the  last  interlineation 
was  made,  "  we  are  ready  to  pass  it  to  be  engrossed." 

"  What  does  that  mean  ?"  asked  John. 

"  To  have  a  fair  copy  made,"  said  Lawrence.  "  In  legis- 
lative bodies,  when  they  have  made  all  the  amendments 
and  alterations  they  wish  in  any  proposed  law,  they  pass 
it  to  be  engrossed — that  is,  to  have  a  fair  copy  made,  in  a 
plain  hand,  so  that  it  can  be  easily  read.  When  this  copy 
is  made  they  have  it  read  again,  and,  if  it  is  all  right,  they 
pass  it  to  be  enacted" 

The  paper  expressing  the  proposed  agreement  between 
Lawrence  and  John,  when  engrossed,  read  as  follows : 

"  I  propose  to  take  John  Wollaston  with  me  to  Europe, 
with  his  father's  and  mother's  consent,  on  the  following 
conditions  in  respect  to  his  studies : 

"1.  That  he  is  to  study  three  hours  every  day,  subject 
entirely  to  my  direction,  Sundays  excepted. 

"  2.  He  is  never  to  intermit  his  studies  on  account  of 
traveling,  whether  on  foot,  by  railway,  or  by  steamer. 

"  3.  He  is  never  to  ask  to  be  excused  from  his  study  on 
account  of  his  not  feeling  well.  If  I  think  he  is  so  unwell 
on  any  day  that  he  ought  not  to  study,  it  will  be  my  duty 
to  say  so. 

"4.  Time  spent  in  reading  attentively  such  books  as  I 
shall  direct,  answering  questions  in  respect  to  what  he  has 
read,  writing  notes  and  abstracts  of  the  same,  and  listening 
to  additional  explanations  from  me,  is  to  be  reckoned  as 


40        THE    LAW   OF   THE    SQUARES    OF   THE    DISTANCES. 

study  hours.  Time  spent  in  writing  letters  or  journals,  or 
in  reading  books  chosen  by  him  for  his  entertainment,  is 
not  to  be  so  reckoned. 

"  5.  The  whole  responsibility  of  keeping  an  account  of 
the  time,  and  of  seeing  to  it  that  he  studies  three  hours 
every  day,  devolves  on  him,  and  not  at  all  on  me. 

"  6.  He  may  gain  to  the  extent  of  one  hour  for  any  par- 
ticular day,  if  he  wishes,  by  studying  over  his  time  on  the 
preceding  ones ;  but  if  he  falls  short  of  his  time  any  day, 
he  can  not  make  it  up  on  succeeding  ones. 

"  7.  Inasmuch  as  all  human  plans  and  arrangements  are 
subject  to  unforeseen  and  unavoidable  difficulties  and  in- 
terruptions, an  allowance  is  made  of  one  day  each  fort- 
night for  failures.  If  the  accidental  failures  do  not  exceed 
this  number,  the  engagement  on  his  part  will  be  under- 
stood to  be  faithfully  kept. 

"  8.  In  case  of  failures  greater  in  number  than  this,  or  in 
case  of  general  remissness  and  neglect  on  his  part  in  the 
fulfillment  of  his  duty  as  herein  stipulated,  there  is  to  be 
ho  penalty  whatever,  except  the  loss  of  credit  which  he 
will  sustain  as  a  young  man  to  be  relied  upon  for  honora- 
bly fulfilling  his  engagements. 

"  LAWRENCE  WOLLASTON. 

"  Agreed  to  by  me, 

"JOHN  WOLLASTON." 

John's  father,  when  these  articles  were  presented  to  him, 
read  them  very  attentively.  John  stood  by  watching  him, 
to  observe  the  effect. 

"The  penalty  does  not  seem  to  be  very  heavy,"  said  he. 

"  Now,  father,"  said  John,  "  I  think  it  is  very  heavy  in- 
deed. I  would  not  lose  my  credit  with  my  cousin  Law- 
rence for  honorably  fulfilling  my  engagements  on  any  ac- 
count whatever." 


STUDY    HOURS.  43 

Mr.  Wollaston  was  glad  to  hear  John  say  this,  and,  after 
some  farther  consideration  and  reflection,  it  was  decided 
that  John  should  go.  And  it  was  under  the  operation  of 
this  agreement  that  John  had  half  an  hour  more  of  study 
to  provide  for  before  he  went  to  bed,  on  the  evening  of  his 
return  from  his  visit  to  the  Tower,  as  described  at  the  com- 
mencement of  the  chapter. 

But  I  have  occupied  so  large  a  portion  of  this  chapter 
in  explaining  the  nature  of  the  agreement  between  Law- 
rence and  John,  and  the  circumstances  under  which  it  was 
made,  that  the  explanation  of  the  law  of  the  squares  of  the 
distances  must  go  over  to  the  next.  I  shall,  however,  let 
the  title  stand ;  we  shall  come  to  the  subject  in  due  time. 


44  CANDLES   TOO   TALL. 


CHAPTER  V. 

CANDLES    TOO    TALL. 

*'  I  WISH  they  would  not  have  such  tall  candles  and  can- 
dlesticks in  our  rooms,"  said  John,  as  he  took  his  seat  at 
the  table.  "  The  light  is  away  up  in  the  air,  and  I  want  it 
down  here  on  the  table,  where  I  am  going  to  write." 

So  saying,  John  began  arranging  his  books  and  papers 
on  the  table,  looking  up,  at  the  same  time,  with  an  expres- 
sion of  dissatisfaction  on  his  countenance,  toward  the  light. 
He,  however,  made  no  more  complaint,  but  said, 

"  I  am  going  to  do  my  half  hour's  study  first,  and  after- 
ward write  in  my  journal.  I  want  to  do  the  hardest  first." 

"I  advise  you  to  write  in  your  journal  first  to-night," 
said  Lawrence ;  "  I  have  a  particular  reason,  which  I  will 
explain  to  you  by-and-by." 

John  was  quite  inclined  in  all  cases  to  follow  Lawrence's 
advice,  as  he  had  always  found  his  "particular  reasons'1' 
very  satisfactory.  So  he  wrote  for  half  an  hour  in  his  jour- 
nal, while  Lawrence  sat  near,  in  a  large  arm-chair,  reading 
the  papers. 

As  John  shut  up  his  journal  and  prepared  to  commence 
his  half  hour's  study,  he  looked  up  at  the  tall  candles 
again. 

"  It  would  have  done  just  as  well,"  said  he,  "if  these  can- 
dles had  only  been  half  as  high,  and  then  I  should  have 
had  twice  as  much  light." 

'•'•Four  times  as  much,"  said  Lawrence. 

"  Twice  as  much,"  said  John ;  "  they  would  have  been 
twice  as  near,  and  so  would  have  given  twice  as  much 
light." 


SIMILAR,  IN    A    GEOMETRIC    SENSE.  45 

"  Being  twice  as  near,"  said  Lawrence,  "  would  make 
them  give  four  times  as  much  light.  This  is  a  case  in 
which  the  law  of  the  squares  of  the  distances  comes  in. 
The  square  of  two  is  four." 

Lawrence  explained  this  principle  to  John  as  follows : 

"Light,  as  we  all  know,  spreads  itself  in  both  directions 
as  it  recedes  from  the  luminous  point — that  is,  laterally, 
which  means  from  side  to  side,  and  also  up  and  down.  If 
it  spread  only  laterally,  then  the  same  light  would,  at 
double  the  distance,  fall  on  double  the  space,  and  would 
consequently  be  weakened  one  half.  But  it  spreads  in  the 
other  direction  also — that  is,  up  and  down ;  so  that  at 
twice  the  distance  it  will  spread  over  four  times  the  space." 

"  That  is  curious,"  said  John. 

"Yes,"  said  Lawrence, "and  it  is  more  curious  still,  as  it 
is  only  a  single  case  of  a  universal  law.  The  two  surfaces 
that  the  same  portion  of  light  from  a  candle  would  shine 
upon  at  different  distances  are  similar,  in  the  geometrical 
sense.  Do  you  know  what  the  wrord  similar  means,  in  a 
geometrical  sense  ?" 

John  said  he  supposed  it  meant  alike,  or  somewhat 
alike. 

"It  means  exactly  alike  inform?  said  Lawrence, "  with- 
out any  regard  to  size.  Thus  an  egg  and  a  ball  are  simi- 
lar, in  common  language,  being  both  rounded,  but  they  are 
not  similar  in  the  geometrical  sense,  because  they  are  not 
exactly  alike  in  form.  A  globe  made  to  represent  the 
earth,  if  it  was  made  a  perfect  sphere,  would,  in  common 
parlance,  be  similar  to  the  earth.  It  would  be  made,  in 
fact,  expressly  in  resemblance  of  it,  but  it  would  not  bb 
similar  in  a  geometrical  sense,  for  the  earth  is  not  a  perfect 
sphere. 

"  So  with  surfaces.  Two  kites  of  exactly  the  same  size 
and  nearly  the  same  shape  would  be  similar,  in  common 


46  CANDLES   TOO    TALL. 

language,  while,  on  the  other  hand,  if,  of  these  two  kites  of 
exactly  the  same  shape,  one  was  only  a  little  toy  an  inch 
long,  and  the  other  were  six  feet  long,  we  should  not  ordi- 
narily say  that  they  were  similar.  We  should  say  that 
they  were  very  different.  In  a  geometrical  sense,  however, 
they  would  be  similar,  while  the  two  that  differed  in  form, 
however  slightly,  though  of  the  same  size,  would  not  be 
similar. 

"  Now,"  continued  Lawrence, "  if  we  cut  a  square  hole 
in  a  piece  of  paper,  and  let  a  light  shine  through  it  upon  a 
card,  or  sheet  of  paper,  for  a  screen,  held  behind  it,  the 
bright  spot  made  on  the  screen  will  be  similar  to  the  open- 
ing, provided  the  screen  is  always  held  square  to  the  light. 
The  size  of  the  bright  spot  would,  however,  be  very  differ- 
ent at  different  distances,  and  the  law  of  increase  would  be 
as  the  squares  of  the  distances— that  is,  at  twice  the  dis- 
tance it  would  be  four  times  as  great;  at  three  times  the 
distance,  nine  times  as  great;  at  five  times  the  distance, 
twenty-five  times  as  great,  and  so  on  in  all  cases." 

"  I  mean  to  try  it,"  said  John. 

So  saying,  he  rose  from  his  seat,  and,  procuring  a  card, 
cut  a  small  square  hole  in  the  middle  of  it.  He  then  put 
one  of  the  candles  away  in  the  closet,  reserving  the  other 
to  form  the  source  of  light.  The  hole  which  he  made  in 
the  paper  was  about  an  inch  square. 

He  then  put  the  reserved  candle  on  the  floor,  and  near  it 
placed  a  chair.  On  the  chair  he  placed  a  big  book,  on  one 
end,  in  such  a  manner  that  he  could  slip  the  card  between 
the  leaves  at  the  other  end,by  which  ingenious  contrivance 
the  card  was  supported  at  about  the  height  of  the  candle. 
He  placed  the  book  so  that  the  card  should  be  at  the  dis- 
tance of  a  foot  from  the  light,  and  then  held  a  sheet  of 
white  paper  at  the  distance  of  another  foot.  He  found,  as 
Lawrence  had  said,  that  the  bright  spot  was  two  inches  in 


DIVERGING   EATS.  47 

dimension  each  way,  making  the  spot  illuminated  by  the 
light  on  the  sheet  four  times  as  large  as  the  hole  through 
which  the  light  came. 

"We  might  know  that  it  must  be  so,"  said  Lawrence, 
"  since  the  rays  of  light  proceed  in  straight  lines,  and  so 
diverge  from  each  other  at  the  same  rate  at  every  distance. 
It  follows  from  this  that,  since  that  portion  of  rays  which 
pass  through  the  square  hole  have  diverged  from  each 
other  one  inch  in  passing  one  foot  from  the  source — and 
those  that  pass  through  the  hole  must  do  exactly  that — 
they  will  diverge  from  each  other  two  inches  in  passing 
through  two  feet.  This  will,  of  course,  make  the  bright 
spot  twice  as  long,  and  also  twice  as  wide,  for  the  diver- 
gence is  the  same  in  both  dimensions,  and  thus  the  bright 
spot  will  be  four  times  as  large  as  the  opening  through 
which  the  light  passed  to  make  it.  By  the  same  reasoning, 
if  the  distance  were  three  feet,  it  would  be  nine  times  as 
large,  for  the  bright  spot  would  contain  three  rows  of 
spaces  as  large  as  the  opening,  and  there  would  be  three 
spaces  in  each  row.  If  the  distance  were  five  feet,  the  il- 
luminated space  would  be  twenty-five  times  as  large  as  the 
opening.  And  so  in  all  cases.  The  space  illuminated  by 
any  particular  portion  of  the  light  from  any  point  will  be 
as  the  square  of  the  distance ;  and  as  the  intensity  of  the 
light,  supposing  that  none  of  it  is  lost,  would  be  dimin- 
ished just  in  proportion  to  its  diffusion,  the  intensity  upon 
any  given  space  will  be  inversely  as  the  square  of  the  dis- 
tance:'' 

The  principle  is  the  same,  whatever  is  the  form  of  the 
opening  through  which  the  light  shines,  whether  square, 
or  round,  or  of  any  irregular  figure.  "Whatever  the  shape 
of  the  opening  may  be,  the  surface  that  it  illuminates  will 
be  of  the  same  shape — that  is,  mathematically  similar,  and 
it  must  be  enlarged,  so  far  as  it  is  enlarged  at  all,  in  two 


48  CANDLES   TOO    TALL. 

dimensions— that  is,  in  what  the  mathematicians  call  a  do« 
plicate,  or  double  ratio. 


ENLARGEMENT  AS  THE   SQUARES  OF  THE   DISTAKOE8. 

Thus,  in  the  engraving,  if  the  distance  from  S  to  M  is 
twice  as  great  as  from  S  to  m,  M  will  be  doubled  in  two 
dimensions,  and  will,  consequently,  be  four  times  as  great 
as  m. 

John  was  much  interested  in  the  experiment  which  he 
had  made,  and  still  more  in  the  general  statement  of  the 
law  which  it  illustrated.  It  is,  indeed,  very  useful  to  know 
this  law,  as  our  action  in  certain  cases  will  be  much  influ- 
enced by  it.  And  few  persons,  unless  they  have  had  in- 
struction on  the  subject,  are  aware  of  it.  It  is  true  that 
every  body  knows  that  the  nearer  we  are  to  the  light  the 
better  we  can  see;  but  it  is  not  every  body  that  knows 
how  much  better — that  is,  every  one  is  not  aware  that  by 
diminishing  the  distance  one  half  between  the  light  and 
his  book,  he  makes  the  brightness  of  it  upon  the  page  four 
times  as  great  as  it  was  before. 

A  gentleman  who  had  occasion  to  travel  much  in  country 
places  where  he  often  found  it  difficult  to  obtain  a  good  light 
for  certain  work  of  writing  which  he  had  to  do,  had  a  flat 


THE  PORTABLE  CANDLESTICKS.  49 

tin  box  made  of  an  oval  form,  about  four  inches  by  three, 
with  a  socket  for  a  short  candle  near  one  end  of  it  on  the 
inside.  He  also  had  a  paper  shade,  which  could  be  fixed 
to  the  candle,  to  throw  the  light  down  upon  his  paper.  By 
this  arrangement  his  light  was  brought  within  six  inches 
of  his  paper,  and  as  the  effect  was  nearly  doubled  by  the 
reflection  from  the  inner  surface  of  the  shade,  which  was 
white,  his  one  candle  flame  threw  nearly  as  strong  a  light 
upon  his  paper  as  eight  candles  would  have  done  at  the 
ordinary  height  of  one  foot.  It  would  have  given  as  much 
light  as  four  candles  without  the  shade,  on  the  principle 
above  explained  of  the  law  of  the  squares  of  the  distances. 
The  cover  of  the  box  was  made  of  the  same  form  with 
the  bottom  of  it,  with  a  socket  in  it,  also  near  one  end,  By 
this  arrangement  the  sockets  did  not  interfere  with  each 
other  when  the  cover  was  put  on,  and  the  gentleman,  if  the 
light  from  one  candle,  near  as  he  brought  it  to  the  paper, 
was  not  enough,  could  at  any  time  have  two,  the  box  serv- 
ing as  one  candlestick,  and  the  cover  as  the  other.  The 


PRACTICAL  RESULTS. 

c 


50  CANDLES   TOO   TALL. 

two  would,  of  course,  give  him  as  strong  a  light  on  his  pa* 
per  as  sixteen  candles  at  the  ordinary  height  would  have 
done. 

Lawrence  talked  with  John  about  the  law  of  the  squares 
of  the  distances  for  some  time,  showing  him  that  it  applied 
to  all  cases  of  the  emanation  of  any  force  from  a  centre, 
such,  for  example,  as  heat  and  gravitation ;  and  this  for  the 
simple  reason  that  the  force,  or  influence,  whatever  it  might 
be,  in  receding  from  the  centre,  was  expanded  in  two  di- 
mensions, length  and  breadth,  and  so  the  surface  within 
which  any  given  portion  of  it  was  included  was  enlarged 
in  two  dimensions,  which  caused  the  surface  to  increase 
not  simply  as  the  distance,  but  as  the  square  of  the  dis- 
tance. And  as  the  intensity  of  the  influence  would  be  di- 
minished just  in  proportion  as  it  was  diffused  over  a  great- 
er space,  the  intensity — that  is,  the  force  at  any  one  point, 
would  be  inversely  as  the  square  of  the  distance. 

This  principle  of  the  very  great  difference  in  the  bright- 
ness of  the  light  at  different  distances  from  the  source  of 
it — a  difference  far  greater  than  one,  without  understand- 
ing the  principle,  would  suppose — is  of  great  importance 
for  all  who  have  to  do  any  work  by  artificial  light,  as,  in 
many  cases,  by  diminishing  their  distance  from  the  light, 
they  can  gain  a  much  greater  advantage  than  they  would 
at  first  imagine. 

There  is  another  principle,  also,  which  it  is  very  impor- 
tant to  understand,  and  that  is  the  illumination  of  the  pa- 
per, or  the  page,  or  whatever  else  it  is  that  the  light  shines 
upon,  depends  not  merely  upon  the  distance,  but  also  upou 
the  angle  at  which  the  rays  fall. 

This  will  be  plainly  seen  by  the  engraving  on  the  opposite 
page,  which  shows  that  when  the  same  book  is  held  oblique- 
ly,  as  it  is  at  "the  left,  it  receives  but  half  as  much  light  as 
when  it  is  at  right  angles  to  the  rays,  as  shown  on  the  right. 


WASTED    LIGHT. 


51 


Lawrence,  moreover,  explained  to  John  that  this  same 
principle  of  the  effect  of  an  increase  in  two  dimensions,  in 
respect  to  any  quantity,  had  a  very  wide  application.  It 
applied,  in  fact,  to  all  similar  surfaces — that  is,  similar  in  a 
geometrical  sense.  If,  for  instance,  we  have  two  rooms, 
and  one  is  twice  as  long  as  the  other,  but  is  of  exactly  the 
same  shape — that  is,  if  it  is  twice  as  great  in  all  its  other 
dimensions,  it  will  take,  not  twice  as  much,  but  four  times 
as  much  carpet  to  carpet  it.  A  person,  without  reflection, 
might  have  said  that  it  would  have  taken  twice  as  much ; 
but,  with  a  little  consideration,  we  see  that  if  it  had  been 
twice  as  long  and  only  just  as  wide,  it  would  have  required 
double  the  quantity  of  carpeting,  but,  being  twice  as  long 
and  twice  as  wide  both,  it  will  take  four  times  as  much. 

It  makes  no  difference  what  the  shape  of  the  two  sur- 
faces may  be,  provided  that  they  are  of  similar  shapes.  A 
boy  has  a  kite  a  foot  long.  He  wishes  to  make  one  of  the 
same  shape  two  feet  long.  It  will  require  four  times  as 
much  paper.  If  he  requires  his  new  kite  to  be  three  times 
as  long  as  the  other,  and  every  thing  in  proportion,  it  will 
require  nine  times  as  much  paper. 

So  with  the  covering  of  a  ball.  There  will  be  four  times 
as  much  leather  in  the  covering  of  a  foot-ball  ten  inches  in 
diameter  as  there  would  be  in  one  of  five  inches ;  for  the 
square  of  five  is  twenty-five,  and  the  square  of  ten  is  one 
hundred,  and  one  hundred  is  four  times  twenty-five. 

It  is  true  that  the  diameter  of  the  balls  are  not  lines  in 


52  CANDLES   TOO   TALL. 

the  covering,  but  that  makes  no  difference.  The  areas  of 
surfaces  are  as  the  squares  of  any  corresponding  lines— 
that  is,  any  lines  bearing  the  same  relation  to  the  two  sur- 
faces compared. 

Lawrence  explained  these  things  to  John,  who  listened 
with  close  attention,  and  asked  many  questions,  and  at 
length  said, 

"  Now  take  your  pencil  and  write  what  I  shall  dictate  to 
you,  to  be  copied  into  your  book  of  notes." 

So  John  took  his  pencil,  and  Lawrence  dictated  as  fol- 
lows : 

"  FUNDAMENTAL  PRINCIPLE. 

"  In  all  cases  of  force  or  influence  of  any  kind  radiating 
from  a  centre,  and  not  intercepted  on  its  way,  the  intensity 
'at  any  point  is  inversely  as  the  square  of  the  distance  from 
the  centre." 

"  And  now,"  said  John,  after  he  had  written  this, "  it  is 
high  time  for  me  to  begin  my  half  hour's  study." 

"  But  your  half  hour's  study  is  over,"  said  Lawrence. 

"Over?"  said  John,  surprised. 

"  I  think  so,"  said  Lawrence.    "  Let  me  see." 

So  saying,  he  took  out  his  watch  and  said,  "Yes,  and  ten 
minutes  more.  Listening  to  instructive  explanations  from 
me,  you  know,  is  to  be  counted  for  study,  if  you  listen  at- 
tentively and  try  to  understand  them." 

"  Good !"  said  John,  in  a  tone  expressive  of  great  exulta- 
tion ;  "  I  thought  I  should  have  half  an  hour  more  of  arith- 
metic before  I  could  go  to  bed ;  but  now  I  can  go  to  bed 
at  once,  for  I  am  tired  and  sleepy." 

So  saying,  he  put  away  his  books  and  papers,  and  pre- 
pared to  go  to  his  room. 

"  But,  Lawrence,"  said  he,  "  what  was  the  particular 


JOHN'S  STUDIES.  53 

reason  you  had  for  wishing  me  to  write  in  my  journal 
first?" 

"  It  was  because  I  was  going  to  explain  to  you  the  law 
of  the  squares  of  the  distances  in  relation  to  radiation  for 
your  study  this  evening,  and  I  thought  you  would  like 
your  journal  work  done  first." 

"  Yes,"  said  John ;  "  I  ana  very  glad  that  you  planned 
it  so." 


54  INTENSITY    OF    LIGHT. 


CHAPTER  VL 

INTENSITY    OF   LIGHT. 

THE  art  of  measuring  the  intensity  of  light  5s  called 
photometry.  The  word  comes  from  the  Greek  word  pho- 
tos, which  means  of  light,  and  the  word  metros,  measure- 
ment. On  the  same  principle,  the  word  photometer  would 
mean  a  light-measurer,  just  as  thermometer  is  a  heat-meas- 
urer, and  barometer  a  weight-measurer,  and  dynamometer 
a  strength -measurer,  from  Greek  words  meaning  heat, 
weight,  and  strength. 

Some  very  curious  devices  have  been  contrived  for  meas- 
uring the  comparative  intensity  of  different  lights.  In  some 
of  thes-e  devices  the  observation  is  made  by  examining  the 
shadows  cast  by  the  two  sources  of  light  to  be  compared. 
How  this  is  done  is  shown  by  the  engraving.  There  is  a 
stand  with  an  upright  rod  (m)  fitted  to  it,  and  beyond  the 
rod  a  screen,  made  usually  of  a  plate  of  ground  glass,  to 
receive  the  shadows.  Any  white  surface  would  answer 
well  enough  for  such  a  screen,  but  ground  glass  is  found 
to  possess  some  peculiar  advantages  for  this  purpose. 

The  two  lights  to  be  compared  are  placed  at  a  distance 
from  the  upright  rod  on  the  side  opposite  to  the  screen,  so 
as  to  cast  the  shadows  a  and  d  upon  it. 

In  the  engraving  the  sources  of  light  are  a  lamp  (L)  and 
a  candle  (B).  The  shadows  seem  to  be  of  nearly  the  same 
intensity.  If,  on  careful  examination,  they  are  found  to  be 
as  nearly  as  possible  alike,  and  if  the  lamp,  as  would  seein 
to  be  the  case,  is  nearly  twice  as  far  from  the  screen  as  the 
candle,  then  it  would  show  that  the  light  from  the  lamp 


THE    PHOTOMETER. 


55 


COMPARISON    OF   SHADOWS. 


would  be  nearly  four  times  as  great  as  that  from  the  can- 
dle. Of  course,  by  exactly  measuring  the  two  distances 
and  squaring  the  numbers  expressing  them,  the  exact  ratio 
would  be  ascertained. 

It  would  be  found,  in  using  this  instrument,  that  if,  in- 
stead of  a  lamp  at  A,  candles  of  the  same  kind  as  the  one 
at  B  are  used,  and  if  the  distance  of  A  from  the  bar  m, 
which  intercepts  the  light,  is  made  double  that  of  B,  there 
must  be  four  candles  at  A  to  make  the  shadows  equal. 


LAW   VEBIFIED. 


There  have  been  various  other  methods  devised  of  meas- 
uring the  comparative  intensity  of  light.  One  more  I  will 
describe. 


56 


INTENSITY    OF   LIGHT. 


It  is  represented  in  the  engraving,  where  you  see  on  the 
left  a  screen  similar  to  the  one  in  the  last  figure,  being 
made  of  a  plate  of  ground  glass  set  in  a  frame.  From  the 
centre  of  the  plate  there  extends  forward  a  shade  or  screen 
of  black  pasteboard,  which  divides  the  ground  glass  plate 
into  two  parts,  and  confines  the  light  coming  from  each  of 
the  two  sources  that  are  to  be  compared  to  its  own  side  of 
the  screen.  In  the  engraving,  the  light  shining  on  one  side 
of  the  screen  is  seen  to  come  from  a  jet  of  gas,  while  that 
on  the  farther  side  is  the  light  of  a  candle.  On  the  table, 
in  lines  extending  from  the  glass  to  the  lights,  are  scales 
of  inches,  or  other  equal  divisions,  by  which  the  distances 
of  the  lights  from  the  glass  respectively  can  be  at  once 
determined. 

Thus  one  half  of  the  glass  plate  is  illuminated  by  one 
light,  and  the  other  by  the  other,  and,  by  looking  at  the 
two  parts  from  the  outer  side,  a  very  exact  comparison 
can  be  made  between  them.  One  or  the  other  of  the 
lights  must  be  moved  until  the  two  illuminations  are  pre- 


TESTING  ILLUMINATING  GAS.  57 

cisely  equal,  and  then,  by  observing  the  distances  at  which 
the  two  lights  are  placed,  and  squaring  the  numbers  repre- 
senting them,  we  get  their  relative  intensities. 

Another  instrument  still,  which  helps  to  show  how  many 
methods  have  been  devised  to  accomplish  this  purpose,  is 
known  as  Ritchie's  Photometer. 


BITCniE'8  PI1OTOM 


The  engraving  shows  it  in  section.  It  consists  of  a  box 
(a  5),  with  openings  on  the  opposite  sides  for  the  admission 
of  light  from  the  two  sources  that  are  to  be  compared.  In 
the  centre,  above,  is  a  conical  tube,  open  at  the  top  at  d. 
Here  the  eye  of  the  observer  is  to  be  placed  to  compare 
the  effects  of  the  two  lights,  which  shine  upon  two  slopes 
of  white  paper,  e/and  e  g,  which  come  together  at  e.  One 
light  or  the  other  is  to  be  moved  until  the  degree  of  il- 
lumination produced  by  them  upon  the  paper  is  the  same. 
The  intensity  of  the  radiance,  then,  from  the  two  sources 
will  be  in  proportion  to  the  squares  of  their  distances  from 
the  centre  of  the  box. 

Instruments  constructed  on  these  principles,  but  quite 
complicated  in  their  details,  are  fitted  up  in  gas-works  to 
determine  the  quality  of  the  gas.  In  France  the  intensity 
of  the  light  is  estimated  by  comparing  it  with  that  fur- 
nished by  a  certain  amount  and  quality  of  oil  burning  at 
a  certain  rate  per  hour,  and  in  England  the  standard  of 
C2 


58  INTENSITY    OF   LIGHT. 

comparison  is  the  light  furnished  by  a  certain  kind  of  can- 
dle. The  lamp  or  the  candle  is  placed  upon  one  scale  of  a 
balance,  with  the  proper  weight  in  the  other  scale.  The 
gas-burner  is  placed  by  the  side  of  it,  and  the  issue  of  gas 
is  so  adjusted  as  to  make  the  two  lights  equal,  as  shown 
by  the  photometer  placed  near.  Both  lights  are  then  al- 
lowed to  burn  until  the  scale  containing  the  lamp  or  can- 
dle rises,  showing  that  the  prescribed  amount  of  oil  or  of 
spermaceti  has  been  consumed.  The  gas  is  then  shut  off, 
and  the  metre  shows  how  much  gas  has  been  consumed. 
By  this  means  the  quantity  of  light  which  the  gas  affords 
per  cubic  foot  is  easily  computed. 

Photometers,  besides  being  useful  in  determining  the 
light-giving  power  of  different  kinds  of  candles  and  differ- 
ent qualities  of  gas,  have  also  been  employed  in  comparing 
the  light  coming  from  various  other  natural  and  artificial 
sources.  Those  who  have  made  these  observations  have 
come  to  the  conclusion  that  the  light  of  the  sun  is  equal 
to  that  of  between  five  and  six  thousand  of  the  standard 
candles,  when  placed  at  the  distance  of  eighteen  inches ; 
that  is,  that  to  throw  a  light  equal  to  the  full  blaze  of  the 
sun  upon  a  sheet  of  paper  would  require  the  combined 
power  of  no  fewer  than  five  to  six  thousand  candles  placed 
at  the  distance  mentioned.  Of  course  it  would  be  practi- 
cally impossible  to  place  that  number  of  candles  so  that 
their  light  could  be  combined.  The  experiment  only  shows 
what  number  would  be  necessary  if  the  combination  were 
possible. 

As  for  the  light  of  the  moon,  even  when  full,  every  one 
knows  that  it  is  vastly  inferior  to  that  of  the  sun,  but  few 
are  aware  how  very  much  inferior  it  is.  The  experiments 
of  scientific  men  with  photometers,  and  the  computations 
which  they  have  made  from  their  observations,  vary  con- 
siderably in  their  results,  as  was  to  have  been  expected, 


INTENSITY   OF   SUNLIGHT.  59 

but  the  average  of  them  makes  the  light  of  the  sun  about 
jive  hundred  thousand  times  as  great  as  that  of  the  full 
moon  on  the  brightest  nights.  Of  course  this  can  only  be 
considered  as  an  approximate  result,  as  it  would  be  impos- 
sible, with  the  means  yet  devised,  to  estimate  such  enor- 
mous differences  of  intensity  between  two  lights  with 
much  accuracy. 


60  CANDLES   AND   LAMPS. 


CHAPTER  VH. 

CANDLES   AND   LAMPS. 

THEKE  is  a  very  near  and  intimate  relation  between  heat 
and  light.  Both  come  together  from  the  sun,  and  both  are 
subject,  in  many  respects,  to  the  same  laws.  In  other  re- 
spects, the  modes  of  action  which  they  present  are  striking- 
ly different.  The  prevailing  opinion  among  scientific  men 
at  the  present  day  is,  that  the  phenomena  of  heat  and  of 
light  are  produced  by  the  same  agent,  modified  in  its  ac- 
tion in  some  mysterious  way,  the  secret  of  which  has  not 
yet  been  discovered. 

We  shall  see,  in  another  chapter,  the  curious  relation 
which  heat  and  light  bear  to  each  other,  as  they  come  to 
us  together  in  the  radiance  of  the  sun. 

One  of  the  most  striking  differences  between  heat  and 
light  is,  that  heat  can  be  absorbed  by  any  substance  and 
afterward  given  out  again  slowly,  but  light,  apparently,  is 
not  subject  to  this  mode  of  action,  except  in  a  few  special 
cases,  and  in  these  only  to  a  very  limited  extent.  If  you 
put  a  brick  or  any  other  substance  in  the  rays  of  a  hot 
sun,  or  before  a  bright  fire,  it  will  absorb  the  heat,  and 
then,  if  afterward  you  take  it  to  a  cool  place  and  hold 
your  hand  before  it,  you  will  feel  the  heat  which  it  has 
absorbed  radiating  from  it  and  warming  your  hand ;  but 
if  you  take  it  into  a  dark  place,  your  eye  will  not  detect 
any  luminous  radiance  from  it — that  is,  there  will  be  no 
evidence  to  the  senses  that  it  absorbed  light  as  well  as 
heat,  so  as  afterward  to  emit  it.  But  perhaps  we  can  not 
certainly  infer,  from  the  fact  that  our  senses  do  not  detect 


EMISSION    OF  LIGHT  AND   HEAT.  61 

any  such  radiance,  that  there  can  not  be  any.  It  is  con- 
ceivable, certainly,  that  the  two  kinds  of  radiance  may  be 
absorbed  and  afterward  emitted  together,  and  that  the 
hand  is  much  more  sensitive  to  the  one  kind  than  the  eye 
is  to  the  other ;  in  other  words,  that  the  radiance,  acting 
as  heat,  will  produce  the  sensation  of  warmth  in  the  nerves 
of  feeling  while  its  intensity  is  yet  low,  and  yet  will  not, 
acting  as  light,  produce  the  sensation  of  vision  in  the  nerves 
of  sight  until  its  intensity  is  very  high. 

However  this  may  be  in  the  case  of  radiance  of  low  in- 
tensity, we  know  that,  when  the  radiance  is  of  high  in- 
tensity, the  heat  and  light  bear  a  very  intimate  relation  to 
each  other,  so  much  so  that  the  degree  of  heat  is  expressed 
often  by  the  kind  and  intensity  of  the  light  that  is  emitted. 
Blacksmiths  and  machinists  say  "red  hot"  and  "white  hot" 
to  indicate  different  degrees  of  temperature,  and  also  "cher- 
ry-red" and  a  "low  red  in  the  dark." 

There  is  a  curious  difference  between  solids  and  gases 
at  high  temperatures  in  respect  to  their  power  of  emitting 
light.  When  any  substance  is  so  intensely  hot  that  it  emits 
bright  light,  we  say  that  it  is  incandescent  /  when  a  gas  is 
incandescent,  it  forms  flame.  Young  persons,  often,  in  look- 
ing at  a  flickering  flame  blazing  up  from  the  fire,  or  at  that 
rising  from  a  candle  or  a  lamp,  wonder  what  it  is.  Now  it 
is  simply  incandescent  gas — a  kind  of  inflammable  air  called 
hydrogen  gas,  which,  in  burning — that  is,  in  combining  with 
the  oxygen  of  the  air — is  heated  to  such  a  degree  as  to  be- 
come incandescent. 

Burning  is  simply  a  chemical  action.  It  is  usually  the 
combining  of  some  combustible  with  the  chemical  sub- 
stance called  oxygen.  There  are  certain  very  curious  con- 
siderations connected  with  the  fact  that  so  much  heat  is 
developed  by  the  combination  of  oxygen  with  combustible 
substances,  but  I  have  not  space  to  explain  them  here.  All 


62  CANDLES   AND   LAMPS. 

that  is  necessary  to  enable  the  reader  to  understand  what 
I  am  going  to  say  about  light  is,  that  combustion  is  a  pro- 
cess that  develops  great  heat,  and  that  the  intensity  of  the 
heat  depends  in  a  great  measure  on  the  rapidity  and  abun- 
dance of  the  supply  of  oxygen. 

The  intensity  of  the  light  which  is  developed  by  the  heat 
depends,  in  a  great  measure,  on  the  substance  heated  con- 
sisting of  solid  particles,  for,  at  the  same  temperature,  the 
particles  of  a  solid  substance  are  found  generally  to  emit 
a  stronger  light  than  those  of  a  gaseous  one.  But  then,  on 
the  other  hand,  certain  gaseous  substances  emit  a  greater 
degree  of  heat  in  combustion  than  most  solid  ones. 

It  results  from  this  that,  in  order  to  have  an  intense 
light,  one  way,  at  least,  would  be  to  have  a  gaseous  sub- 
stance to  burn  in  order  to  produce  the  heat,  and  some 
solid  particles,  or  solid  substance,  to  be  heated  by  it,  to  af- 
ford the  light. 

This  is  very  simple,  and  yet  this  is  the  philosophy  of  the 
modes  generally  adopted  to  produce  artificial  light  and  to 
increase  the  intensity  of  it. 

Take  a  common  lamp  or  candle,  for  example,  burning 
with  a  naked  flame — that  is,  without  any  glass  chimney. 
The  tallow,  or  wax,  or  spermaceti,  or  paraifine,  or  oil,  or 
kerosene,  or  whatever  other  similar  combustible  is  used,  is 
composed  chiefly  of  carbon  and  hydrogen,  and  all  these 
substances  are  called,  accordingly,  hydrocarbons.  When 
they  are  burned  in  the  wick  of  the  candle  or  lamp,  the  hy- 
drogen, which  is  the  gas,  burns  and  produces  a  great  heat, 
and  the  floating  particles  of  carbon,  which,  though  exceed- 
ingly minute  —  too  minute  altogether  to  be  seen  by  the 
naked  eye— are  yet  solid,  become  intensely  heated,  and  it 
is  they  that  emit  the  bright  light. 

Hydrogen,  burning  alone,  emits  a  very  feeble  light.  We 
sometimes  see,  in  a  wood  fire,  faint  blue  flames  here  and 


COMBUSTION   AND    ILLUMINATION.  63 

there  which  have  very  little  illuminating  power.  These 
are  usually  flames  of  hydrogen,  and  are  produced  in  places 
where,  for  some  accidental  reason,  hydrogen  only  for  a  few 
minutes  happens  to  issue.  They  would  be  found  to  be  very 
hot  if  we  had  any  way  of  testing  their  temperature,  but 
they  would  afford  but  a  very  feeble  light  to  write  by  if 
by  anv  means  we  could  bring  one  of  them  to  the  table. 

The  flame  of  an  alcohol  lamp  is  almost  entirely  a  hydro- 
gen flame,  and,  though  it  is  very  hot,  it  gives  very  little 
light,  on  account  of  there  being  no  solid  particles  of  car- 
bon in  it  to  be  intensely  heated  by  it  and  to  emit  their 
superior  light. 

It  is  all  the  better,  on  this  account,  for  the  purposes  that 
the  alcohol  lamp  is  used  for — namely,  for  producing  heat; 
for,  if  there  were  solid  particles  of  carbon  in  the  flame,  just 
so  far  as  the  force  of  the  heat  should  be  expended  in  heat- 
ing them  so  as  to  give  light,  there  would  bo  less  heat  for 
the  water,  or  the  coffee,  or  the  blowpipe,  or  for  any  of  the 
other  heating  purposes  for  which  the  flame  was  used. 

And  then,  besides,  the  floating  particles  of  carbon  in  the 
flame,  if  intercepted  by  any  substance  before  they  are  con- 
sumed, blacken  it,  or,  as  AVC  say,  smoke  it.  If  you  hold  a 
piece  of  cold  iron  or  any  other  such  substance  in  the  flame 
of  a  candle  or  lamp,  it  becomes  smoked,  as  we  say.  The 
philosophy  of  this  is,  that  a  great  many  of  the  floating  par- 
ticles of  carbon  are  intercepted  by  the  cold  substance  be- 
fore they  are  consumed,  and  so  become  attached  to  it,  and 
blacken  it.  This  proves  that  the  particles  of  carbon  are 
really  in  the  flame  all  the  time,  though  we  do  not  see  them, 
nor  see  any  indications  of  their  presence,  except  in  the  in- 
creased brightness  of  the  flame,  in  consequence  of  their  be- 
ing themselves  heated  intensely  hot  in  it  and  in  process 
of  being  consumed.  But  by  holding  the  iron,  or  any  cold 
substance,  in  the  flame,  wre  at  once  cool  all  the  particles 


64  CANDLES   AND   LAMPS. 

that  come  in  contact  with  it,  and  so  stop  their  combustion, 
and  then  their  true  character  is  at  once  revealed. 

Very  often  a  portion  of  the  particles  of  carbon  escape 
from  the  flame  themselves  without  being  burnt,  and  go  up 
the  chimney  in  the  form  of  a  blue  smoke.  The  white  vapors 
which  are  seen  arising  sometimes  from  a  fire  are  vapors  of 
water  or  steam,  but  the  blue  fumes  are  composed  of  parti- 
cles of  carbon,  some  of  which  escape  out  of  the  chimney 
into  the  air,  while  a  portion  of  them  lodge  upon  the  sides 
of  it,  forming  soot. 

Some  substances  give  out  a  much  greater  quantity  of 
carbon  in  burning  than  others,  as,  for  example,  birch  bark, 
pitch-pine  knots,  and  the  "light-wood,"  so  called,  of  the 
Southern  States.  A  great  portion  of  this  carbon  is  made 
incandescent  in  the  flame,  and  gives  out  great  light.  That 
is  the  reason  why  those  substances  make  such  excellent 
torches.  Of  the  carbon  which  is  thus  made  incandescent 
in  these  flames,  some  is  burned — that  is,  it  finds  oxygen 
enough  to  combine  writh  it  in  the  flame — and  so  disappears 
as  carbon,  and  forms  another  substance.  But  some  of  the 
particles  which  are  made  incandescent — that  is,  red  hot — 
in  the  flame,  and  so  help  to  emit  light,  are  not  burned,  be- 
cause there  is  not  oxygen  enough  for  all.  This  portion, 
then,  escapes  into  the  air,  where  it  cools  and  becomes  black 
again — that  is  to  say,  each  separate  particle  becomes  black ; 
but  generally,  when  it  comes  from  a  common  fire,  being 
more  or  less  mingled  with  a  certain  portion  of  watery  va- 
por, which  is  white,  the  mixture  assumes  a  bluish  hue. 
When,  however,  it  is  not  so  modified — as,  for  instance, 
sometimes  when  issuing  from  the  smoke-pipe  of  a  steamer 
— it  shows,  by  its  very  dark  bluish  color,  what  its  true 
character  is. 

The  cause  of  this  escape  of  carbon  unconsumed  is  that 
the  supply  of  oxygen  for  the  flame  is  insufficient ;  for, 


IMPERFECT   LAMPS. 


whenever  a  particle  of  carbon  becomes  red  hot  in  the  pres- 
ence of  oxygen,  it  immediately  combines  with  it,  and  forms 
another  substance  which  is  entirely  invisible.  There  have 
been  devised  in  modern  times  many  modes  of  furnishing 
supplies  of  oxygen  for  flames  in  a  more  rapid  and  abundant 
manner,  so  as  to  prevent  the 
escape  of  any  unconsumed 
carbon,  but  in  early  times 
no  method  was  known  of 
doing  this.  Indeed,  the  ne- 
cessity or  desirableness  of 
doing  it  was  not  understood, 
for  scarcely  any  thing  was 
known  before  the  middle  of 
the  last  century  in  regard  to 
the  true  nature  of  flame,  or 
of  the  conditions  on  which 
the  greater  or  less  degree 
of  light  which  could  be  de- 
rived from  it  depended. 

Accordingly,  in  early 
times  lamps  were  used,  quite 
artistic  sometimes  in  exter- 
nal form,  but  very  rude  and 
imperfect  in  respect  to  the 
principle  on  which  they  op- 
erated. There  was  no  ar- 
rangement to  facilitate  the 
supply  of  oxygen,  nor  to 
prevent  the  disturbing  and 
cooling  eflect  of  currents  of 
I  air  upon  the  flame,  so  that 
a  faint  and  flickering  light, 
accompanied  by  a  great  deal 


ANCIENT   LAMP. 


66  CANDLES   AND   LAMPS. 

of  unconsumed  carbon  in  the  form  of  smoke,  was  the  cer- 
tain result. 

The  only  light  for  the  streets  of  cities  in  Europe  two  or 
three  hundred  years  ago  was  furnished  by  great  flaming 
and  smoking  torches  carried  in  the  hand.  The  darkness 
at  night,  of  course,  afforded  great  facilities  for  the  commis- 
sion of  all  kinds  of  crime,  and  robberies,  murders,  and  as- 
sassinations increased  to  such  a  degree  that  the  govern- 
ment of  Paris  at  one  time  organized  a  guard  of  armed  men 
to  patrol  the  streets  in  search  of  the  criminals,  lighting  their 
way,  of  course,  by  the  only  kind  of  illumination  they  then 
knew  how  to  produce,  viz.,  that  of  blazing  and  smoking 
torches,  which  the  link-man  carried  before  them  in  his  hand. 

The  true  remedy  for  this  state  of  things  was  to  dispel 
the  darkness  which  occasioned  it  by  devising  some  way  to 
increase  the  brightness  of  the  light  which  could  be  given 
by  a  flame,  and  then  lighting  the  streets  by  placing  a  fixed 
burner  of  this  increased  brightness  at  every  corner. 

The  first  method  of  attempting  to  do  this  was  by  means 
of  a  reflector  placed  behind  the  flame,  so  as  to  throw  all 
that  part  of  the  sphere  of  light  issuing  from  the  flame, 
which  would  naturally  go  back  toward  the  wall,  where  it 
was  not  wanted,  forward  into  the  street.  But  very  soon 
the  attention  of  scientific  men  began  to  be  turned  to  the 
question  whether  the  intensity  of  the  light  itself  could  not 
be  increased  by  increasing  the  intensity  of  the  heat  pro- 
duced, and  then  promoting  the  rapidity  of  the  combustion 
by  a  more  complete  and  rapid  supply  of  oxygen.  There 
would  evidently  be  a  double  advantage  in  this,  for,  by  fur- 
nishing a  full  supply  of  oxygen,  all  the  carbon  would  be 
consumed,  instead  of  being  allowed  in  part  to  escape  un- 
consumed as  smoke,  and  then,  moreover,  the  particles  which 
were  consumed  would  be  raised  to  a  higher  intensity  of 
heat,  and  so  would  become  more  highly  luminous. 


TUB  POLICE   OF  OLD  TIMES. 


BLOWING  THE   FLAME    OUT.  69 

Now,  in  the  case  of  an  ordinary  fire  of  wood  or  of  coal, 
the  way  to  increase  the  supply  of  oxygen  is  to  blow  it  with 
the  bellows ;  that  is,  to  send  in,  by  means  of  the  bellows,  a 
rapid  current  of  air  containing  the  necessary  oxygen.  But 
it  is  a  curious  circumstance  that,  while  the  blowing  of  a 
solid  fire  makes  it  burn  all  the  brighter,  blowing  the  flame 
of  a  candle  puts  it  out.  What  is  the  reason  of  this  ? 

Fully  to  understand  the  reason,  it  must  be  observed  that 
blowing  a  fire  has  three  different  effects  upon  it — first,  to 
supply  oxygen  to  it,  and  so  make  it  burn  faster ;  secondly, 
to  cool  it ;  and,  thirdly,  by  its  mechanical  impulse,  to  blow 
the  burning  fuel  away.  In  the  case  of  the  blacksmith's 
forge,  only  the  first  of  these  effects  is  produced  to  any  con- 
siderable extent.  The  current  of  air  supplies  oxygen  to 
increase  the  combustion,  which  greatly  increases  the  heat. 
It  brings  coolness  too,  and  so  prevents  the  heat  from  be- 
coming as  great  as  it  would  be  if  the  bellows  could  blow 
hot  air  instead  of  cold ;  but  the  influence  of  the  greater 
supply  of  oxygen  in  promoting  the  combustion  is  vastly 
greater  in  increasing  the  heat  than  the  cooling  effect,  even 
in  the  coldest  winter  day,  is  in  diminishing  it.  And  as  to 
the  third  effect,  the  coals  being  solid  and  comparatively 
heavy,  the  current  of  air  is  not  strong  enough  to  blow 
them  away. 

If,  however,  we  imagine  that  the  blast  was  so  powerful 
as  to  blow  the  coals  from  the  forge  all  over  the  black- 
smith's shop,  the  fire  would  be  put  out  by  it  at  once — that 
is,  as  soon  as  the  individual  coals  had  time  to  go  out  in 
their  new  places,  scattered  over  the  bench  and  floor.  If 
the  coals  were  very  small,  this  would  be  very  soon ;  and 
if  we  imagine  each  one  of  them  to  be  no  larger  than  a  par- 
ticle of  dust,  the  extinguishment  would  be  almost  instan- 
taneous. 

This  is  precisely  what  happens  when  we  blow  out  a  can- 


70  CANDLES    AXD    LAMPS. 

die.  The  flame  is  a  burning  or  incandescent  gas,  with  ex- 
tremely minute  particles  of  solid  carbon,  infinitely  finer 
than  any  visible  dust,  pervading  it.  When  you  blow  it, 
therefore,  with  a  strong  puff  of  air,  the  whole  incandescent 
gaseous  mass  is  blown  away,  and  is  instantly  cooled  below 
the  point  of  combustion  ;  in  other  words,  it  goes  out. 

If  there  is  at  the  time,  however,  a  portion  of  the  wick  in- 
candescent, as  there  usually  is,  that,  as  it  can  not  be  blown 
away,  remains  burning,  and  the  more  you  blow  upon  it  the 
brighter  it  glows,  until,  as  fast  as  successive  portions  of  it 
become  loosened  and  driven  off,  the  incandescent  mass  is 
diminished;  and  as  the  coolness  of  the  blast  prevents  the 
combustion  from  extending  itself  to  portions  below,  the 
wick,  as  well  as  the  flame,  is  soon  entirely  extinguished. 

So  much  for  the  philosophy  of  blowing  out  a  candls. 


FUKNACE    BLOWERS.  71 


CHAPTER 

THE    ABGAND    BURNER. 

1$  view  of  the  facts  and  explanations  given  in  the  last 
chapter,  it  is  easy  to  understand  that  one  way,  at  least,  of 
attempting  to  increase  the  light  given  out  by  any  flame  is 
to  continue  some  mode  of  increasing  the  supply  of  oxygen 
for  it  without  dispersing  or  scattering  the  burning  materials / 
in  other  words,  of  "  blowing"  the  candle  or  fire  without 
blowing  it  out. 

It  was  a  Swiss  inventor  named  Argand  who  first  con 
trived  to  do  this,  and  the  contrivance  which  he  devised 
is  called  the  Argand  burner  to  this  day. 

But,  in  order  that  you  may  clearly  understand  the  prin- 
ciples of  his  invention,  I  must  first  say  that  there  are  two 
ways  of  "  blowing"  fires  in  furnaces  and  forges :  one  by 
driving  in  the  current  of  air  by  the  force  of  propulsion  be- 
low, and  the  other  by  drawing  it  in,  by  the  force  of  ex- 
haustion in  the  chimney  above. 

The  former  is  effected  by  means  of  bellows,  and  some- 
times by  another  contrivance  called  a  fan-blower,  by  either 
of  which  a  strong  blast  is  forced  into  the  fire  at  the  grate. 
In  some  furnaces  where  a  very  great  heat  is  required,  the 
air  is  heated  before  it  is  driven  into  the  furnace,  so  that 
the  full  effect  of  the  additional  supply  of  oxygen  may  be 
secured  without  any  diminution  being  caused  by  the  cool- 
ness of  the  current  of  air. 

The  latter  of  the  modes  above  mentioned — that  is,  the 
drawing  of  air  in  by  the  force  of  exhaustion  in  the  chim- 
ney above,  is  effected  by  making  the  chimney  very  tall. 


72  THE    ABOARD   BURNER. 

The  air  within  the  chimney,  being  heated,  is  light  and  buoy- 
ant, and,  of  course,  the  taller  the  chimney,  the  more  buoy- 
ancy there  is,  and  the  greater  the  draft— that  is,  the  faster 
the  air  is  "  drawn  in,"  as  we  usually  express  it,  though  the 
real  mode  of  operation  is  that  the  pressure  of  the  atmos- 
phere above  the  fire  being  taken  off,  in  part,  by  the  buoy- 
ancy of  the  hot  air  in  the  chimney,  the  air  is  forced  in  to 
the  fire  by  the  atmospheric  pressure  which  acts  on  the  or- 
ifice below. 

Now  Argand's  plan  was  to  furnish  the  increased  supply 
of  oxygen  to  the  fire  in  the  flame  of  the  lamp  or  candle  by 
"  drawing  it  in"  from  below  by  means  of  a  chimney,  and 
he  also  conceived  the  thought  of  bringing  in  the  current  in 
the  middle  of  the  flame  instead  of  around  the  outside  of  it. 

Argand,  as  has  already  been  said,  was  a  Swiss.  He  was 
of  quite  humble  origin,  but  he  received  a  scientific  educa- 
tion, and  in  the  earlier  part  of  his  life  he  was  engaged  very 
successfully  in  the  southern  part  of  France  in  connection 
with  industrial  occupations,  in  which  his  scientific  knowl- 
edge, and  especially  his  knowledge  of  chemistry,  were  of 
great  service. 

His  attention  was  called,  while  thus  employed,  to  the 
subject  of  light,  especially  for  use  in  manufacturing  and 
other  such  establishments;  for  in  those  days — near  the 
close  of  the  last  century — there  was  nothing  in  use  for  ar- 
tificial light  but  such  naked,  smoking,  and  flickering  flames 
as  are  given  out  by  common  lamps,  torches,  and  flambeaux. 
His  knowledge  of  chemistry  showed  him  that  the  reason 
why  the  flames  were  not  bright  was  the  scantiness  of  the 
supply  of  air,  which  could  only  reach  the  flame  on  the  out- 
side. It  had  been  discovered  some  time  previously  that 
an  ordinary  flame  was  hollow — being  bright  only  on  the 
outer  surface  of  it — as,  of  course,  it  must  be,  as  in  the  case 
of  such  a  flame  there  is  no  access  to  the  air  within. 


THE  TROUBLES  OP  THE  INVENTOR.  73 

So  Argand  set  himself  at  work  to  contrive  a  way  by 
which  to  admit  air  to  the  centre  of  the  flame ;  and  after  a 
great  many  experiments  and  a  great  deal  of  contrivance, 
he  succeeded  in  producing  a  cylindrical  wick  which  was  to 
be  inclosed  between  two  concentric  tubes,  with  an  opening 
at  the  bottom  of  the  inner  tube  for  a  supply  of  air.  He 
also  provided  suitable  mechanism  for  raising  and  lowering 
the  wick5>and  fitted  a  sheet-iron  chimney  over  it  to  in- 
crease the  draft  up  through  the  inner  tube. 

He  made  his  chimney  of  sheet-iron,  because  in  those  days 
they  had  no  means  of  making  glass  chimneys  that  would 
stand  so  great  a  heat  without  breaking.  Of  course  it  was 
necessary  to  place  the  chimney  so  that  the  lower  edge  of 
it  should  be  just  above  the  upper  edge  of  the  flame,  in  or- 
der that  the  light  might  not  be  intercepted. 

Not  long  after  this  the  glass-makers  contrived  to  make 
'glass  chimneys  which  would  stand  great  heat  provided 
they  were  heated  gradually,  and  then  Argand's  invention 
was  complete. 

But  the  invention,  great  as  its  value  has  proved  to  be 
for  mankind,  was  the  source  to  the  unhappy  inventor  of  it 
of  nothing  but  trouble  and  sorrow.  He  became  involved 
in  disputes  and  lawsuits  with  other  men,  especially  with  a 
Frenchman,  whose  name  is  spelled  Quinquct,  and  is  pro- 
nounced, as  nearly  as  can  be  represented  by  English  sym- 
bols, Kaingkay.  Quinquet,  it  would  seem,  drew  Argand's 
idea  from  him  in  conversation,  or,  at  least,  obtained  such 
glimpses  of  it  as  enabled  him  to  produce  a  lamp  of  the 
same  character;  and  he  harassed  and  thwarted  Argand  in 
alibis  attempts  to  obtain  what  would  correspond  to  a. pat- 
ent right  to  it  at  the  present  day.  Argand  went  to  En- 
gland, and  there  was  more  successful.  His  invention  was 
adopted  in  that  country,  and  was  recognized  as  his,  and 
the  contrivance  is  called  the  Argand  burner  there  and  iu 
D 


74  THE    ARGAND    BURNER. 

America  to  this  day.  But  in  France  the  name  of  Quinquet 
finally  carried  the  day,  and  a  lamp  there,  with  a  burner  on 
this  principle,  is  always  called  a  Quinquet. 

Argand  was  worn  out,  mind  and  body,  by  his  long-con- 
tinued disappointments  and  troubles,  and  when  he  was 
only  a  little  past  middle  life  he  returned  to  the  home  of  his 
childhood  in  Switzerland,  poor,  disheartened,  and  miserable, 
and  died  in  the  imbecility  and  wretchedness  of.  a  prema- 
ture old  age. 

And  now,  nearly  a  century  since  his  death,  they  who  un- 
derstand these  facts,  after  they  have  been  reading  for  an 
hour  in  the  evening  by  the  bright  light  which  his  simple 
and  beautiful  contrivance  has  given  them,  sometimes  pay  a 
brief  tribute  to  hiss  memory  by  observing  for  a  moment  in 
silence  the  brilliant  and  beautiful  effect  produced  by  the 
double  current  of  air,  intensified  in  its  action  by  the  draft 
of  the  chimney,  and  then  saying  to  themselves, "  Poor  Ar- 
gand !" 


LUMINIFEROUS    ETHER.  75 


CHAPTER  IX. 

INTERMINGLING    OF    UNDULATIONS. 

As  has  already  been  stated,  there  are,  or,  rather,  have 
been,  two  theories  in  respect  to  the  physical  nature  of 
light — one,  that  it  consists  in  the  emanation  of  streams  of 
exceedingly  minute  particles,  which  fly  through  the  air 
with  inconceivable  swiftness,  having  in  some  mysterious 
way  the  power  of  passing  through  glass  and  all  transpa- 
rent bodies ;  and  the  other,  that  it  consists  in  a  vibratory 
or  undulatory  motion  in  a  subtle  medium,  which,  in  order 
to  have  a  name  for  it,  has  been  called  ether.  The  existence 
of  this  ether  is  only  imaginary,  however,  as  nothing  is  di- 
rectly known  in  respect  to  it,  and  it  is  only  supposed  to 
exist,  as  the  sole  means  that  we  can  conceive  of  to  render 
the  transmission  of  luminous  undulations  possible. 

It  seems,  however,  as  has  already  been  said,  very  diffi- 
cult to  conceive  of  the  possibility  of  undulations  in  such 
infinite  number  and  variety  as  must  be  moving  at  every 
point  in  space,  if  this  theory  is  true,  meeting,  and  encoun- 
tering, and  crossing  each  other  without  in  the  least  degree 
interfering  with  or  disturbing  each  other's  motions.  Still 
we  can  not  say  that  this  would  be  impossible.  There  is 
complete  and  positive  proof  that  sound  is  produced  by  vi- 
brations in  the  air;  and  yet,  on  a  calm  summer  morning, 
we  can,  by  listening,  hear  a  great  many  different  sounds, 
all  clear  and  distinct,  and  each  produced  by  its  own  undu- 
lations, coming  through  the  same  medium  with  all  the  rest, 
and  each  without  being  sensibly  disturbed  by  the  others. 
We  can  hear  the  songs  of  two  or  three  different  birds,  the 


76  INTERMINGLING    OF    UNDULATIONS. 

talk  of  children  at  play,  the  whistle  of  a  distant  locomo- 
tive, the  bark  of  a  dog,  the  crowing  of  a  cock,  the  chirp  of 
a  cricket,  and  the  faint  tones  of  the  bell  in  the  village  spire, 
miles  away.  Though  we  can  not  well  attend  to  all  these 
sounds  at  once,  we  can  hear  them  all,  and,  if  we  select  any 
one  to  listen  to  specially,  we  can  hear  it  distinctly  and 
clearly,  showing  that  the  undulations  which  produce  it 
come  to  us  through  the  air  undisturbed  by  the  undulations 
of  all  the  rest,  which,  however,  they  must  necessarily  trav- 
erse at  every  conceivable  angle  on  the  way. 

John  had  a  curious  opportunity  to  observe  the  phenom- 
enon of  undulations  crossing  each  other  without  serious 
interference  one  evening  while  he  was  with  Lawrence  in 
London.  It  was  in  St.  James's  Park. 

There  are  several  large  parks  in  London  where  people 
go  for  recreation  and  amusement.  The  nearest,  and,  in 
some  respects,  the  most  attractive  of  these,  is  St.  James's 
Park.  This  park  is  smaller  than  any  of  the  others,  but  it 
is  nearer  the  heart  of  the  town,  and  so  is  more  accessible 
to  large  numbers  of  people.  The  queen's  palace  and  gar- 
dens are  near  it  on  one  side,  the  houses  of  Parliament,  and 
Westminster  Abbey,  and  the  Horse  Guards  (the  great  head- 
quarters of  the  army)  on  another,  and  the  streets  all  around 
it  are  lined  with  gay  shops  and  elegant  residences. 

In  the  park  is  a  long  and  beautiful  lake,  crossed  in  the 
middle  by  a  suspension  bridge.  There  are  walks  along 
the  margin  of  the  lake,  arid  chairs  for  people  who  wish  to 
sit  and  rest,  and  beds  and  borders  of  flowers,  and  swans, 
and  ducks,  and  other  kinds  of  swimming  birds  upon  the 
water,  and  on  pleasant  summer  evenings  the  grounds  are 
full  of  ladies,  and  gentlemen,  and  children  walking  about 
and  amusing  themselves  in  various  ways. 

One  evening,  about  an  hour  before  the  sun  went  down, 
as  Lawrence  and  John  were  walking  together  in  one  of  the 


AN  EXPERIMENT  BY  THE  DUCKS.  77 

streets  in  that  part  of  the  town,  on  their  way  home  from 
Westminster  Abbey,  where  they  had  been  spending  an 
hour  wandering  about  through  the  aisles,  and  transepts, 
and  chapels,  looking  at  the  monuments  and  other  curious 
things  to  be  seen  there,  Lawrence  stopped,  and,  pointing 
to  a  side  street,  said, 

"I  am  going  to  turn  off  here  and  go  into  the  park. 
There  is  an  experiment  that  I  am  going  to  have  performed 
there  for  you." 

"  Who  is  going  to  perform  it  ?"  asked  John. 

"A  couple  of  ducks,"  said  Lawrence,  gravely. 

John  laughed,  but  he  turned  very  readily  in  the  direc- 
tion which  Lawrence  indicated. 

They  soon  entered  the  park  by  a  ponderous  iron  gate, 
and,  after  walking  a  little  way  over  a  broad  gravel  walk 
well  filled  with  parties  of  ladies  and  gentlemen,  and  boys 
and  girls,  going  to  and  fro,  and  separated  on  each  side 
from  the  shrubberies,  and  lawns,  and  beds  of  flowers  by  an 
open  iron  fence,  they  came  to  a  suspension  bridge  lead- 
ing over  a  narrow  portion  of  the  lake.  They  crossed 
this  bridge,  and  then,  after  proceeding  a  little  farther, 
they  found  a  row  of  chairs,  which  were  placed  by  the 
side  of  the  walk  and  facing  the  water.  They  took  their 
seats  in  two  of  these  chairs,  and  looked  out  upon  the  little 
lake. 

Immediately  before  them,  across  the  walk,  was  a  band 
of  green,  with  large  trees  here  and  there  upon  it,  so  near, 
however,  that  their  branches  intermingled.  Under  these 
trees  there  was  a  view  of  the  water,  with  ducks  swimming 
here  and  there  over  the  surface  of  it.  The  sheet  of  water 
was  not  very  wide,  and  beyond  it,  the  farther  shore  was 
covered  with  groves  of  trees  and  thickets  of  shrubbery. 

"Well,"  said  John,  as  soon  as  they  were  seated  and  had 
viewed  the  landscape  before  them  for  a  moment,  "and 


INTERMINGLING    OF   UNDULATIONS. 


TI1E   DUCKS   ON   THE   LAKE. 


what  is  the  experiment  that  the  ducks  are  going  to  per- 
form?" 

" It  is  an  experiment  on  the  crossing  of  undulations" 
said  Lawrence.  "You  see  these  ducks  are  swimming  about 
in  all  directions,  and  each  one,  as  he  parts  the  water  with 
his  breast  and  his  paddling  legs,  makes  two  lines  of  waves, 
or  undulations,  which  diverge  from  each  other  as  they  re- 
cede behind  him,  or,  rather,  as  he  advances  and  leaves 
them.  There  is  something  very  curious  in  the  laws  of 
motion  that  govern  the  formation  and  the  spread  of  these 
lines;  but  I  am  not  going  to  say  any  thing  about  that 
now,  but  only  to  have  you  see  what  the  effect  is  when  two 
of  these  lines  of  waves  cross  each  other.  You  -would  think, 
in  such  a  case,  that  they  would  disturb  and  destroy  each 
other,  as  one  would  suppose  the  undulations  or  vibrations 
of  light  would  do.  But  you  will  see,  when  we  get  a  good 
chance — that  is,  Avhen  two  ducks  happen  to  come  along 
side  by  side,  so  that  the  lines  of  waves  cross  each  other — 
that  there  is  much  less  interference  than  one  would  sup- 


FIRST   AND    SECOND    CLASS    CHAIRS.  79 

pose,  and  that  the  different  lines  go  on  after  the  crossing 
much  as  before." 

Just  at  this  time  a  neatly  but  plainly  dressed  woman 
came  along  the  walk,  having  a  little  leathern  bag  hanging 
by  her  side.  She  advanced  to  Lawrence,  and  held  out  her 
hand  for  the  money  to  pay  for  the  chairs. 

"  How  much  ?"  asked  Lawrence. 

"  Two  pence,"  said  the  woman — or,  rather,  as  she  pro- 
nounced it,  "tuppence." 

Lawrence  gave  her  the  money,  and  she  went  away. 

"  I  thought  they  were  only  a  penny  apiece,"  said  John. 

"  That's  for  the  common  chairs,"  said  Lawrence.  "  We 
have  taken  arm-chairs,  and  so  have  made  ourselves  first- 
class  people. 

"We  might  as  well  have  taken  the  common  chairs,"  said 
John ;  "  they  would  have  been  just  as  good  for  us  to  sit 
here  and  see  the  ducks." 

"Exactly,"  said  Lawrence;  "only  then  we  should  have 
marked  ourselves  as  second-class  people.  Every  thing  is 
managed  in  England  on  the  principle  of  social  classifica- 
tion. When  other  people  don't  class  you,  you  have  to 
class  yourself.  Americans  almost  always  prefer  to  pay 
the  difference,  rather  than  make  themselves  second-class 
people.  But  the  English  don't  care  so  much ;  they  are 
used  to  such  distinctions." 

"I  don't  care  much,"  said  John. 

"The  difference  does  not  amount  to  much  in  such  a  case 
as  this,"  said  Lawrence,  "but  it  is  worth  thinking  of  some- 
times, as,  for  instance,  on  a  long  journey.  When  we  go 
to  Paris,  you  can  save  a  pound  or  two,  I  suppose — which 
would  be  equal  to  five  or  ten  dollars — by  going  second 
class,  and  so  have  that  amount  to  spend  for  apparatus  or 
books  in  Paris." 

"Then  I'll  do  it,"  said  John,  jumping  up  suddenly  from 


80  INTERMINGLING    OF    UNDULATIONS. 

his  chair.     "I'll  certainly  do  it ;  I'd  as  lief  go  second  ciasa 
as  not." 

"Very  well,"  said  Lawrence.  "But  now  look  at  the 
ducks;  there  are  two  coming  now  directly  before  us  in 
just  the  right  position." 

The  two  ducks  that  Lawrence  referred  to  were  twin* 
ming  along  nearly  side  by  side,  at  a  short  distance  from 
the  shore,  and  the  little  line  of  waves  which  passed  off 
from  the  left  side  of  one  crossed  that  which  came  from 
the  right  side  of  the  other,  but  the  two  lines  seemed 
scarcely  to  interfere  with  each  other  at  all.  They  ap- 
peared to  go  on  after  the  crossing,  each  on  its  way,  as  if 
it  had  been  very  little  disturbed  by  the  other. 

This  is  only  one  among  the  innumerable  cases  occurring 
in  nature  which  show  the  possibility  of  the  coexistence  of 
different  vibrations  among  the  same  set  of  particles — that 
is,  in  the  same  substance — with  a  degree  of  independence 
of  each  other  which,  without  proof  from  experiment,  we 
should  have  thought  impossible.  John  was  very  much 
surprised  to  see  how  little  disturbed  the  diverging  lines 
of  waves  made  by  the  two  ducks  were  in  crossing  each 
other.  It  is  true  that  afterward,  when  he  saw  several 
ducks  swimming  this  way  and  that,  in  all  directions,  a 
good  deal  of  irregular  commotion  was  produced  on  the 
surface  of  the  water ;  but  this  apparent  confusion  seemed 
to  be  caused  quite  as  much  by  the  difficulty  of  following 
with  the  eye,  and  separating  by  the  mind,  all  the  differ- 
ent lines,  as  by  any  actual  interference  in  the  undulatory 
actions. 

The  case  of  different  sounds  coming  through  the  air  to 
the  ear,  which  has  already  been  referred  to,  is  another  in- 
stance of  the  coexistence  of  different  vibrations  in  the 
same  substance,  each  preserving  unimpaired  its  own  dis- 
tinctive character.  So,  when  a  bell  is  struck— especially 


HARMONICS.  81 

if  it  is  a  large  and  heavy  bell — besides  the  general  vibra- 
tion which  emits  the  principal  and  dominant  tone,  there 
are  always  a  great  many  others  which  blend  with  the 
principal  one,  and  combine  with  it  in  the  effect  of  produc- 
ing the  general  sound.  These  subordinate  tones  are  called 
the  harmonics.  It  is  so  with  a  musical  string  or  wire ;  it 
has  its  harmonics  as  well  as  its  principal  sound,  and  they 
all  come  together  through  the  air  to  the  ear  without  in- 
terfering with  each  other.  So,  when  a  band  of  one  hun- 
dred performers  is  playing,  it  is  wonderful  to  think  what 
an  immensely  complicated  mass  of  vibrations,  produced  by 
so  many  kinds  of  instruments  and  playing  so  many  differ- 
ent parts,  must  come  to  the  ear  through  the  air  at  the  same 
time ;  and  though  many  of  them  may  mingle  and  blend  so 
as  to  produce  a  delightful  harmony,  they  do  not  disturb 
or  derange  each  other  at  all,  for  if  they  did  so  the  result 
would  be  only  a  discordant  noise. 

"Such  cases  as  these,"  said  Lawrence,  "make  it  just  pos- 
sible for  us  to  conceive  that  the  emanations  of  light,  mov- 
ing constantly,  as  they  do,  with  such  amazing  velocity,  and 
in  every  direction  through  the  starry  heavens,  without  in- 
terfering with  each  other  at  all,  may  be  propagated,  as  sci- 
entific men  now  suppose,  by  undulations  or  oscillations  ir» 
a  very  subtle  and  highly  elastic  medium. 

"At  any  rate,"  he  added,  "this  much  is  certain,  that 
these  emanations  are  propagated,  in  some  way  most  mys- 
terious to  us,  in  right  lines,  diverging  in  every  direction 
from  the  centre ;  that  they  expand  in  two  dimensions  as 
they  advance,  and  so  each  portion  of  them  occupies  a 
space  that  increases  as  the  square  of  the  distance,  and,  of 
course,  that  the  intensity  on  a  surface  of  given  magnitude 
diminishes  as  the  square  of  the  distance,  or,  as  the  mathe- 
maticians say,  is  inversely  as  the  square  of  the  distance ; 
and,  finally,  that  these  rays  are  wholly  invisible  to  us,  ex- 
D2 


82  INTERMINGLING    OF   UNDULATIONS. 

cept  so  far  as  they  are  intercepted  in  their  passage  and  re* 
fleeted,  so  as  to  come  to  us  and  enter  our  eyes,  where  they 
form  an  image  on  the  retina  and  produce  vision." 

When  Lawrence  had  said  this,  he  took  out  his  watch, 
looked  at  it,  and  said, 

"Well,  we  have  been  here  looking  at  the  ducks,  and  talk- 
ing about  the  intermingling  of  vibrations  and  undulations, 
about  twenty  minutes,  or  twenty-five.  Shall  we  call  this 
time  study  hours  or  not  ?" 

"That  is  just  as  you  say,"  replied  John.  "I've  learned 
something,  at  any  rate,  and  something  that  I  did  not  know 
before." 

"And  you  have  listened  attentively  to  it,"  replied  Law- 
rence. "I  think  it  would  be  fair  to  consider  it  study  hours. 
And  there  is  one  thing  more  that  I  should  like  to  explain, 
which  will  take  about  five  minutes,  if  you  are  not  too  tired, 
and  that  will  just  make  up  the  half  hour." 

John  said  he  was  not  too  tired,  and  would  like  very  well 
to  make  up  the  half  hour.  So  Lawrence  explained  that, 
although  the  emanations  of  light,  whether  they  were  really 
of  the  nature  of  undulations  or  not,  did  not  appear,  in  ordi- 
nary cases,  to  interfere  with  each  other  at  all,  however  nu- 
merous and  complicated  their  intercrossings  might  be,  that 
still  there  was  an  optical  phenomenon  which  was  called  in- 
terference, although,  strictly  speaking,  it  was  not  interfer- 
ence in  the  common  sense,  since  both  of  the  radiations  in 
these  cases  produced  its  own  full  effect,  though  the  two 
effects  combined  produced  a  somewhat  remarkable  result. 

Lawrence  explained  the  principle  by  asking  John  to  im- 
agine that  two  ducks  were  swimming  over  the  water  in 
such  directions  that  the  undulations  should  not  cross  each 
other,  but  should  follow  each  other  in  long  lines  exactly 
parallel. 

"Now  we  may  suppose,"  he  added,  "that  these  lines  are 


INTERFERENCE    OF    LIGHT.  83 

more  or  less  near  to  each  other.  We  may  imagine  them 
to  coincide  exactly — that  is,  that  the  crest  of  the  wave  of 
one  shall  coincide  exactly  with  the  crest  of  the  wave  of  the 
other,  and  the  hollow  Avith  the  hollow — and  that  by  this 
coincidence  the  height  of  the  wave  would  be  increased,  and 
also  the  depth  of  the  hollow,  so  that  the  two  undulations 
combined  should  form  a  united  one  of  double  intensity. 
We  can  also,  on  the  other  hand,  imagine  that  the  two  sets 
of  undulations  may  be  separated  from  each  other  by  half 
the  breadth  of  the  tcave,  so  that  the  hollow  of  one  should 
just  correspond  with  the  swell  of  the  other,  and  thus  that 
each  should  counteract  the  effect  of  the  other,  and  the  water 
consequently  remain  smooth." 

"Oh, Lawrence,"  said  John,  "it  would  not  be  possible  to 
fit  the  two  lines  of  the  waves  so  exactly  as  that." 

"  True,"  replied  Lawrence ;  "  but  is  it  impossible  to  con- 
ceive of  it  ?" 

"  Xo,"  rejoined  John,  "  I  don't  think  it  is  impossible  to 
conceive  of  it." 

"You  are  right,  probably,"  added  Lawrence,  "in  saying 
that  it  would  be  impossible  to  perform  this  experiment 
with  any  waves  and  by  means  of  ducks,  but  it  can  be  rep- 
resented perfectly  by  artificial  waves." 

"  Artificial  waves  ?"  repeated  John. 

"  Yes,"  replied  Lawrence ;  "  there  is  an  article  of  appa- 
ratus by  which  the  action  and  appearance  of  waves  can  be 
produced  by  means  of  bars  of  wood  rising  and  falling,  so 
as  to  illustrate  the  laws  of  their  motion.  You  turn  a 
crank,  and  a  motion  representing  a  wave  runs  along  the 
machine.  With  this  they  can  show  very  plainly  what  I 
have  been  explaining  to  you.  There  are  two  sets  of  mov- 
ing bars,  or  two  systems,  either  of  which  alone  makes  a  line 
of  waves.  When  they  are  combined  in  a  way  to  make  the 
elevations  and  depressions  of  both  systems  correspond^  the 


84  INTERMINGLING    OF   UNDULATIONS. 

waves  are  of  double  height ;  but  when  they  are  combined 
so  as  to  make  the  elevations  of  one  correspond  with  the 
depressions  of  the  other,  then  there  are  no  waves  at  all. 
The  mechanism  moves,  but  the  surface  of  the  water,  or 
that  which  represents  the  surface  of  the  water,  remains 
smooth." 

"  That's  curious,"  said  John. 

"And  so  with  light,"  added  Lawrence.  "There  is  a  way 
of  contriving  to  make  the  two  sets  of  luminous  undulations 
unite  in  such  a  manner  as  to  produce  darkness.  I  can  not 
explain  to  you  now  how  it  is  done ;  only  remember  that  in 
the  books  it  is  called  interference.  It  is  only  interference, 
however,  in  one  sense.  Strictly  speaking,  neither  interferes 
with  or  hinders  the  real  and  proper  action  of  the  other,  but 
the  two  actions  combined  produce  a  remarkable  result. 

"And  now,"  added  Lawrence,  rising,  "your  five  minutes 
are  out,  and  more  too,  and  it  is  time  for  us  to  go  home. 
Only  you  must  try  to  remember  exactly  what  is  meant 
philosophically  by  interference,  as  the  word  is  used,  in  the 
science  of  light." 

As  Lawrence  and  John  walked  over  the  suspension 
bridge  on  their  return  home,  they  stopped  to  watch  the 
motions  of  the  ducks  and  swans  which  were  swimming 
about  in  one  place  near  the  shore.  John  looked  attentive- 
ly to  see  whether  he  could  detect  any  thing  like  the  phe- 
nomenon of  interference  in  the  optical  sense,  but  he  could 
not. 

"  At  any  rate,"  said  he,  "  I  should  like  to  have  one  of 
those  wave  machines  that  you  described." 

"You  can  buy  one,  perhaps,  when  you  get  to  Paris," 
said  Lawrence,  "  with  the  money  that  you  save  by  going 
second  class." 

John's  father  was  a  very  wealthy  man,  and  was  perfect- 


EARNING   PLEASURES.  85 

ly  willing  to  supply  his  son  with  all  the  money  that  he 
could  judiciously  use,  and  Lawrence  had  full  authority  to 
furnish  John  with  whatever  he  thought  was  judicious.  But 
they  both  had  the  good  sense  to  know  that  a  boy  enjoys 
any  acquisition  that  he  may  make  much  more,  and  feels  a 
more  real  and  substantial  sense  of  property  in  it,  if  he  has 
done  something  to  earn  it  himself  by  some  kind  of  effort, 
or  sacrifice,  or  self-denial.  A  father  who  supplies  his  son 
freely  and  thoughtlessly  with  all  the  money  that  he  wants 
acts  very  unphilosophically.  He  is  as  unphilosophical  as 
he  would  be  in  thinking  that  it  would  do  just  as  well  to 
give  his  boy  money  to  buy  fish  of  a  vendor  going  by,  as  to 
let  him  take  his  rod  and  line  and  go  a  fishing  himself  to 
catch  them. 


86  REFLECTED   AND   TRANSMITTED   LIGHT. 


CHAPTER  X. 

REFLECTED    AND   TRANSMITTED    LIGHT. 

A  GREAT  many  curious  and  beautiful  illusions  are  pro- 
duced by  the  reflection  of  light.  One  of  the  most  remark- 
able of  these  is  the  exhibition  of  pretended  ghosts  and  hob- 
goblins at  places  of  public  entertainment.  John  went  with 
Lawrence  to  witness  one  of  these  exhibitions  at  a  place  of 
instruction  and  amusement  in  London  called  the  Polytech- 
nic Institution,  where  the  whole  process  was  explained.  I 
shall  presently  give  an  account  of  this  visit,  but  in  the 
mean  time,  in  order  that  the  reader  may  clearly  under- 
stand the  nature  of  the  phenomenon,  it  is  necessary  that  he 
should  pay  attention  for  a  moment  to  a  certain  mathemat- 
ical principle. 

If  you  throw  a  ball  from  your  hand  to  the  floor  direcf.li/ 
downward — that  is,  at  right  angles  to  the  floor,  its  tenden- 
cy is  to  rebound  directly  upward — that  is,  to  come  up  as  it 
went  down,  namely,  at  right  angles. 

On  the  other  hand,  if  you  throw  the  ball  somewhat  for- 
ward, so  that  it  shall  strike  the  floor  at  some  distance  be- 
fore you,  it  will,  in  rebounding,  go  still  farther  on.  In  this 
case  the  ball  strikes  the  floor  at  an  oblique  angle,  and,  on 
rebounding,  or  being  reflected,  as  we  might  say,  it  rises  at 
the  same  angle  on  the  other  side. 

It  follows  from  this  that  if  a  boy  and  a  girl  standing  at 
a  little  distance  apart  are  playing  with  a  ball,  and  the  boy 
wishes  to  throw  the  ball  so  that  the  girl  may  easily  catch 
it  on  the  rebound,  he  must  throw  it  so  that  it  shall  strike 
the  floor  as  nearly  as  possible  midway  between  them,  so 
that  it  may  have  a  horizontal  distance  to  rise  in  equal  to 


THE   BOUNCING   BALL. 


87 


that  which  was  occupied  in  its  descent ;  for  this  it  must 
have  if  it  is  to  rise  at  the  same  angle  and  attain  to  the 
same  height. 

There  is  the  same  tendency,  substantially,  when  the  ball 
is  thrown  against  a  perpendicular  wall,  though  in  this  case 
the  eftect  is  modified  to  a  greater  extent  by  the  weight  of 
the  ball. 

In  the  case  of  the  reflection  of  liglit,  the  effect  has  noth- 
ing to  interfere  with  it,  and  the  result  is,  that  universally 
the  angle  at  which  the  light  comes  to  the  reflecting  sur- 
face on  one  side  is  the  same  as  that  at  which  it  leaves  it 
on  the  other,  or,  as  it  is  expressed  scientifically, 

The  angle  of  incidence  is  equal  to  the  angle  of  reflection. 

This  is  one  of  the  fundamental  laws  of  optics. 

The  principle  is  made  plain  by  the  engraving,  where  C 


ANGLES   OF   INCIDENCE    AND    REFLECTION. 


REFLECTED    AND   TRANSMITTED    LIGHT. 


represents  a  small  hole  in  a  shutter  admitting  a  ray  of 
light  from  the  sun  into  a,  darkened  room.  The  light  falls 
upon  a  mirror  lying  horizontally  upon  a  table,  making  with 
the  perpendicular,  A  D,  the  angle  of  incidence,  C  D  A.  The 
ray  is  reflected  in  the  direction  D  B,  as  far  to  the  right  of 
the  perpendicular  as  it  came  in  on  the  left  ;  in  other  words, 
making  the  angle  of  reflection,  A  D  B,  equal  to  the  angle 
of  incidence,  ADC. 

An  instrument  has  been  devised  for  showing  that  these 
angles  are  exactly  equal,  so  far  as  mathematical  principle 
of  this  kind  can  be  shown  by  experiment.  The  next  en- 
graving represents  this  instrument. 

It  consists  of  a  graduated  circle,  in  the  centre  of  which 
the  mirror  is  placed,  as  shown  at  n. 
Attached  to  the  graduated  circle  are 
two  slender  tubes,  A  and  B,  made 
movable  upon  the  arc.  The  distance 
of  each  from  the  central  point  above 
can  be  easily  determined  by  the  grad- 
uation. It  is  found,  in  experimenting 
with  this  instrument,  that  when  one 
of  the  tubes,  A,  is  so  adjusted  on  the 
arc  upon  one  side,  and  the  instrument 
is  so  placed  upon  the  table  that  a  ray 
of  light  from  the  sun  passes  through  the  tube  to  the  mir- 
ror, and  the  other  tube  is  placed  at  the  same  distance  on 
the  other  side,  then,  and  only  then,  will  the  ray,  after  re- 
flection, pass  out  through  the  other  tube,  B. 

In  the  same  manner,  when  the  two  tubes  are  placed  at 
the  same  distance  from  the  upper  middle  point  of  the  arc, 
no  matter  what  the  distance  is,  a  person  looking  through 
one  of  them  will  see  any  small  object  —  a  key,  for  example, 
held  at  the  opening  of  the  other,  showing  that  in  all  cases 
the  angles  of  incidence  and  reflection  are  equal. 


MODE  OF  MEASUREMENT. 


REFLECTING    SURFACES.  89 

Almost  all  substances  reflect  a  greater  or  less  portion  of 
the  light  that  falls  upon  them.  When  a  substance  does 
not  reflect  light  at  all,  it  appears  black.  When  the  surface 
is  very  smooth — as  smooth  as  it  is  when  we  say  it  is  pol- 
ished, and  is  at  the  same  time  plane — then  it  reflects  the 
light  regularly  and  uniformly,  and  we  have  an  image  of 
the  luminous  source  in  it.  This  is  because  a  sufficient  num- 
ber of  the  rays  are  simply  turned  from  their  coui-se  and  di- 
rected toward  the  eye  to  form  vision.  They  move  at  the 
same  angle,  in  relation  to  each  other,  after  they  are  reflect- 
ed as  before,  and  so  enter  the  eye  just  as  they  would  have 
done  had  they  not  been  reflected,  but  only  came  from  a  dif- 
ferent direction. 

But  when,  on  the  other  hand,  the  surface  is  not  polished, 
then  the  portions  of  the  surface  on  which  the  light  falls 
are  broken  and  irregular;  for,  even  though  we  call  it 
smooth,  it  is  not  perfectly  smooth,  and  the  different  por- 
tions of  it  reflect  the  light  each  in  its  own  way.  Thus  the 
light  comes  to  our  eyes  in  a  confused  manner. 

You  can  get  a  clear  idea  of  this  by  first  supposing  that 
a  looking-glass  is  lying  in  the  bottom  of  a  basket,  whole, 
so  as  to  reflect  clearly  and  distinctly  objects  seen  in  it,  and 
then  that  afterward  it  is  broken  into  pieces,  and  the  pieces 
lie  in  confusion  in  the  basket,  all,  however,  right  side  up. 
The  reflections  would,  in  this  last  case,  be  confused.  You 
would  see  a  general  light,  but  no  distinct  image. 

This  is  precisely  what  happens  in  the  case  of  ice.  When 
it  is  whole,  and  has  a  smooth  and  level  surface,  it  reflects 
the  ice  like  a  mirror;  but  when  it  is  broken  up  into  fine 
pieces,  it  presents  a  general  white  appearance,  like  snow, 
and  that  is  all.  Snow  itself  consists  in  minute  sheets  and 
needles  of  ice,  any  one  of  which,  by  itself,  reflects  light  like 
a  mirror ;  but  it  is  too  small  to  reflect  any  complete  object, 
however  minute,  and  so  all  the  reflections  coming  conr 


90  REFLECTED    AND    TRANSMITTED    LIGHT. 

fusedly  together  to  the  eye  produce  the  sensation  of  white- 


There  is  another  thing  very  curious  about  reflection, 
which  it  is  necessary  to  understand,  in  order  that  the  man- 
ner in  which  the  ghost  illusion  is  produced  may  be  fully 
intelligible.  It  is  this,  that  the  light  reflected  from  any 
mirror  into  the  eye  appears  to  come  from  an  object  lying 
in  the  direction  of  the  rays  as  they  enter  the  eye.  This 
must  of  course  be  so,  for  the  vision  is  produced  in  the  eye, 
and  by  the  rays  which  enter  into  it,  and  its  character  is 
of  course  determined  entirely  by  the  character  of  the  rays 
and  the  direction  in  which  they  are  moving  when  they  enter 
it.  In  other  words,  the  impression  made  upon  the  eye  is 
determined  entirely  by  the  condition  of  the  rays  of  light 
after  they  enter  the  organ.  It  can  take  no  cognizance  of 
any  changes  these  rays  may  have  undergone  on  the  way. 

This  is  illustrated  in  the  engraving,  where  a  portion  of 
the  light  from  the  candle  is  reflected  from  the  mirror  and 
enters  the  eye.  But  you  will  see  that  it  enters  the  eye 
from  such  a  direction,  and  with  such  a  degree  of  conver- 
gence— as  is  sho-vn  by  the  upper  arrow — as  if  it  had  come 
from  behind  the  glass.  Of  course  it  produces  such  an  im- 
age in  the  eye,  and  such  a  conception  in  the  mind,  as  if  it 
had  really  come  from  that  position ;  just  as  if  the  glass, 
instead  of  being  a  mirror,  had  been  transparent,  and  the 
light  had  come  from  an  object  behind  it  in  the  position 
and  in  the  light  necessary  to  send  rays  directly  to  the  eye, 
such  as  were  really  sent  to  it  by  the  reflection. 

In  the  engraving,  only  that  portion  of  the  rays  from  the 
candle  are  represented  which  comes  to  the  eye.  Of  course, 
in  fact,  the  radiance,  in  such  a  case,  emanates  from  the 
flame  in  every  direction,  and  some  portion  of  it  is  reflected 
from  every  part  of  the  surface  of  the  glass,  and  is  dispersed 
to  almost  every  part  of  the  room.  It  follows  from  this 


EXPLANATION    OF   VISION. 


f        •..        .1     .',« 


APPARENT   DIRECTION. 


that  an  eye  in  any  place  to  which  the  reflected  rays  come, 
it'  turned  toward  the  mirror,  would  have  an  image  of  the 
candle  formed  within  it  by  means  of  a  different  beam,  and, 
of  course,  the  candle  would  appear  to  each  observer  in  a 
different  place  behind  the  mirror,  namely,  in  a  place  cor- 
responding to  the  course  of  his  own  particular  beam. 

Thus,  if  there  were  twenty  persons  looking  at  the  reflec- 
tion of  the  same  candle  in  the  same  glass,  there  would  be 
twenty  images,  distinct  from  each  other  in  this  sense,  name- 
ly, that  each  would  be  seen  reflected  in  a  different  part  of 
the  glass  from  the  other,  and  would  be  produced  by  a  dif- 
ferent beam  of  light. 

You  must  remember,  then,  that  the  mental  perception 
we  have  of  any  object,  formed  by  the  image  of  it  in  the 
eye,  depends  entirely  upon  the  manner  in  which  the  light 


92  REFLECTED    AND    TRANSMITTED    LIGHT. 

from  it  enters  the  eye,  and  not  at  all  upon  any  changes  in 
direction  or  in  character  that  it  may  have  undergone  be- 
fore it  enters.  This  is  one  of  the  principles  on  which  illu- 
sions are  created,  namely,  by  causing  the  light  to  enter 
the  eye  in  the  way  it  would  enter  if  it  came  really  from 
such  a  source  as  would  correspond  with  the  intended  illu- 
sion. 

There  is  one  other  curious  thing  to  be  observed  in  order 
to  understand  clearly  the  philosophy  of  the  ghost  illusion, 
and  that  is,  that  while  opaque  substances,  such  as  the  met- 
als, will  only  reflect  light,  without  transmitting  any — that 
is,  will  throw  back  what  falls  upon  its  surface,  but  will  not 
allow  any  to  pass  through  its  substance  from  the  other 
side,  transparent  substances,  like  glass  and  water,  will  re- 
flect apart  of  the  light  which  falls  upon  them,  and  allow  a 
part  to  pass  through.  Thus  we  can  look  out  through  a 
window  into  the  street,  and  see  what  is  there,  and  we  can 
also  often  see  the  reflection  of  the  fire,  or  any  other  bright 
object  in  the  room,  on  the  inside.  When  you  look  out 
through  a  window  from  a  room,  and,  still  more,  when  you 
look  into  a  shop  window  from  the  street,  it  often  happens 
that  you  can  not -see  clearly  on  account  of  the  "glare." 
This  glare  is  only  the  reflection  of  light  from  the  glass  on 
your  side  of  it,  which  reflection  prevents  your  seeing  clear- 
ly the  transmitted  light,  or,  rather,  seeing  clearly  the  objects 
on  the  other  side,  which  can  only  be  seen  by  transmitted 
light. 

Thus,  when  you  look  into  any  such  glass — that  of  a  win- 
dow, for  instance,  some  reflected  and  some  transmitted  light 
comes  to  your  eye— that  is,  some  light  from  your  face,  and 
the  other  objects  on  the  same  side  of  the  glass  with  your- 
self, which  is  reflected,  and  some  from  the  objects  on  the 
other  side  which  is  transmitted,  or,  in  other  words,  which 
passes  through :  of  these  two  it  is  the  strongest  that  con- 


WHICH    IS    THE    MOKE    POWERFUL?  93 

quers — that  is,  it  is  the  light  from  those  objects  which  are 
most  strongly  illuminated,  on  whichever  side  they  may  hap- 
pen to  be,  that  makes  the  greatest  impression  on  the  eye. 

Thus  the  objects  in  the  open  air  are  usually  much  more 
brightly  lighted  than  those  in  the  room,  and  so,  in  looking 
out  at  a  window,  we  see  the  trees,  and  the  houses,  and  the 
people  in  the  street,  the  light  from  which  is  strong,  and  is 
transmitted  to  us  through  the  glass,  while  we  do  not  see 
the  objects  in  the  room  reflected  in  the  glass,  because  the 
light  from  them  is  much  feebler ;  and  though  it  is  reflected 
just  the  same,  notwithstanding  that  the  transmitted  light 
is  coming  through,  it  does  not  affect  our  vision  much  in 
presence  of  the  brighter  light  that  is  transmitted. 

But  if  you  hold  a  candle,  a  lighted  match,  or  any  other 
bright  light  before  a  window,  you  see  the  reflection  of  it  at 
onee  in  the  pane,  even  in  the  daytime.  You  can  also  see 
even  a  piece  of  white  paper  reflected  in  a  window-pane, 
unless  there  is  a  very  bright  light  outside. 

So,  in  the  evening,  when  the  room  is  lighted  up,  while  it 
is  nearly  dark  outside,  the  objects  in  the  room  are  then  the 
best  illuminated,  and  the  light  reflected  from  them  over- 
powers altogether,  in  the  effect  produced  on  the  retina  of 
the  eye,  that  coming  from  the  street,  and  it  is  much  more 
difficult  to  see  any  thing  outside  on  account  of  this  reflec- 
tion. You  have  to  hold  your  hands  up  on  each  side  of 
your  face  to  cut  off"  the  light  from  the  objects  in  the  room, 
which  would  otherwise  be  reflected  too  strongly. 

This  explains  the  reason  why  it  is  sometimes,  especially 
in  a  bright  day,  difficult  to  see  plainly  what  is  in  the  shop- 
windows  along  the  street,  especially  if  the  panes  are  of 
plate-glass.  You  see  the  objects  in  the  street  by  the  re- 
flected light  in  the  panes,  because  the  light  from  them  is 
stronger  than  the  light  from  the  objects  within. 

So,  in  the  daytime,  when  the  light  is  stronger  without 


94  REFLECTED    AND    TRANSMITTED    LIGHT. 

than  it  is  within  the  room,  those  in  the  room  can  see  what 
is  passing  in  the  street  very  well,  while  those  in  the  street 
can  not  see  distinctly  what  is  passing  in  the  room.  In  the 
evening,  however,  the  case  is  reversed.  Then,  the  room 
being  better  lighted  than  the  street,  we  have  always  to 
close  the  curtains,  for  otherwise,  the  light  being  greater 
now  within  than  without,  those  who  are  within  can  not  see 
easily  the  objects  without,  while  those  without  can  see 
very  plainly  what  is  passing  within. 

This  principle,  simple  as  it  is,  operates  in  a  thousand  dif- 
ferent ways,  and  intelligent  young  persons,  who  once  un- 
derstand it.  will  take  pleasure  in  discovering  examples  of 
it  which  will  be  continually  coming  under  their  observa- 
tion, if  they  have  only  learned  "  how  to  observe." 

It  is  often,  for  example,  well  illustrated  in  the  case  of 
water.  "Water  is  transparent,  and  the  surface  of  it,  when 
left  in  repose,  makes  plane  and  polishes  itself,  thus  forming 
a  mirror;  so  that  we  can  see  objects  on  the  bottom  by 
light  which  comes  up  through  it,  or  objects  on  the  banks 


BEFLKCT10N    FRC 


REFLECTIONS    FROM    THE    WATER.  95 

by  light  which  is  reflected,  which  we  see  most  distinctly 
depends  upon  what  is  most  brightly  illuminated. 

When  the  water  is  tolerably  deep,  so  that  the  bottom 
receives  little  light,  then  the  reflected  light  predominates 
in  the  eye,  and  we  see,  as  in  the  engraving,  the  buildings, 
and  the  trees,  and  the  grass  along  the  shore,  and  even  the 
forms  of  the  ducks  swimming  on  the  surface,  by  reflected 
light.  The  light,  too,  is  reflected  in  accordance  with  the 
principle  already  explained,  namely,  so  as  to  make  the 
angle  of  incidence  equal  to  the  angle  of  reflection.  The 
course  of  only  two  rays  is  given  in  the  engraving — one 
from  each  of  two  points  in  the  eaves  of  the  buildings,  to 
show  the  principle.  In  truth,  however,  there  would  be  an 
infinite  number  of  rays  from  each  of  these  points,  diverging 
in  every  direction,  and  striking  the  water  at  every  angle, 
all  of  them  being  reflected  on  the  other  side  of  the  perpen- 
dicular to  each,  at  the  same  angle  they  made  in  coming  to 
the  surface.  But  only  those  which  fell  upon  the  water  at 
such  an  angle  as  to  cause  them  to  proceed,  after  reflection, 
to  the  eye  of  the  boy,  would  take  effect  in  his  vision,  and 
he  would  see  the  image  of  the  points  from  which  they  pro- 
ceeded in  the  direction  in  which  they  came  to  him,  as 
shown  by  the  dotted  prolongation  of  the  lines  beneath  the 
water. 

In  the  same  manner,  from  every  other  point  in  the  build- 
ings and  in  the  adjoining  trees  light  would  be  radiated, 
and  would  fall  upon  the  water  at  every  angle,  and  those 
rays  that  fell  at  the  right  angle  to  be  carried,  after  reflec- 
tion, to  the  boy,  would  aid  in  completing  the  picture  of  the 
landscape  in  his  eye.  Thus  he  would  have  an  image  of  the 
entire  scene  upon  his  retina,  the  several  parts  of  it  appear- 
ing as  far  below  the  water  as  the  real  object  was  above, 
except  so  far  as  the  effect  would  be  modified  by  the  higher 
or  lower  portion  of  his  eye. 


96  REFLECTED   AND   TRANSMITTED    LIGHT.     _a=^/* 

If  the  -water  were  perfectly  smooth,  then  the  image  would 
be  perfectly  clear  and  distinct.  But  this  can  never  be,  as 
the  surface  of  water  is  never  perfectly  smooth.  It  is  ruf- 
fled by  slight  movements  of  the  air  in  the  calmest  days. 
In  this  case,  the  ripples  made  by  the  swimming  of  the  ducks 
affect  it  •-  and  even  the  swimming  of  fishes  or  of  frogs,  far 
below,  is  sufficient  to  produce  some  change  in  the  surface, 
and  to  prevent  absolute  repose. 

When  water  is  deep  and  the  bottom  is  dark,  we  see  the 
reflection  most  clearly,  for  then  there  is  very  little  trans- 
mitted light  coming  up  through  to  disturb  it ;  but  when 
the  water  is  shallow,  and  the  objects  on  the  bottom  are 
bright  and  clear,  then  we  see  the  reflection  indistinctly  or 
not  at  all,  for  the  reflected  rays  are  overpowered  by  those 
which  are  transmitted  from  below. 

These  are,  then,  the  fundamental  principles  on  which  the 
ghost  illusion  depends,  namely,  that  any  material  which  is 
at  once  transparent  in  substance,  and  also  plane  and  pol- 
ished upon  its  surface,  like  a  plate  of  glass,  for  example, 
will  transmit  and  also  reflect  light  at  the  same  time,  so  that 
you  can  see  objects  through  it  and  objects  reflected  in  it 
together,  and  the  comparative  distinctness  with  which  you 
see  them  depends  upon  the  comparative  intensity  of  the 
light  with  which  they  are  respectively  illuminated. 


BEGENT   STREET.  97 


CHAPTER  XI. 

SPECTRES   AND   GHOSTS. 

ONE  of  the  principal  cross  streets  of  London  is  Regent 
Street.  It  crosses  the  great  thoroughfares  which  run 
lengthwise  through  the  vast  city,  and  which  are  the  scenes 
of  the  principal  movement  to  and  fro,  being  filled  in  all  the 
business  hours  of  the  day  with  long  lines  of  cabs,  and  om- 
nibuses, and  drays,  and  coaches,  and  carriages  of  all  sorts. 
These  long  thoroughfares,  extending  for  many  miles,  are 
lined  with  shops  of  every  kind;  but  Regent  Street,  which 
crosses  them  in  the  gayest  and  most  fashionable  part  of  the 
town,  is  emphatically  the  street  of  shops,  or  stores  as  we 
call  them  in  America ;  for  the  goods,  and  wares,  and  ob- 
jects of  interest  and  curiosity,  and  the  novelties  of  dress, 
and  articles  for  presents,  and  books,  and  engravings,  are 
more  numerous  and  splendid,  and  more  tastefully  arranged 
behind  the  great  windows  of  plate-glass  here  than  in  any 
other  part  of  London.  If  you  wish  to  buy  any  curious  or 
pretty  thing  to  bring  home  with  you  as  a  souvenir  of  Lon- 
don, there  is  not  a  better  place  to  look  for  it  than  Regent 
Street. 

There  is  one  part  of  the  street  that  is  more  splendid  than 
the  rest.  At  the  lower  end  of  it,  where  it  approaches  the 
region  of  the  houses  of  Parliament,  the  club-houses,  and 
the  palaces  of  the  old  nobility,  it  makes  a  grand  sweep  in 
the  form  of  a  quarter  of  a  circle.  This  part  of  the  street 
is  known,  in  fact,  as  the  Quadrant ;  and  as  the  buildings  on 
each  side  are  uniform  in  architecture  throughout  the  whole 
length  of  it,  and  as  the  shops  are,  if  possible,  more  gay  and 
E 


98  SPECTKES   AND   GHOSTS. 

splendid  than  those  in  other  parts  of  the  street,  the  Quad- 
rant is  quite  a  celebrated  place  for  going  a  shopping  in,  es- 
pecially for  strangers  and  visitors  in  London. 

The  Polytechnic  Institution,  or,  as  it  is  more  commonly 
called  by  abbreviation,  the  Polytechnic,  is  situated  in  Re- 
gent Street,  though  it  is  in  the  straight  part  of  it,  at  a  con- 
siderable distance  from  the  Quadrant.  John  saw  one  day 
an  advertisement  in  the  papers  of  an  entertainment  that 
was  given  there  every  evening,  in  which  the  manner  in 
which  ghost  illusions  are  produced  on  the  stage  was  to  be 
explained,  and  he  felt  a  strong  desire  to  go  and  see  it. 

"  Yes,"  said  Lawrence, "  I  should  like  to  go  too ;  but,  so 
far  as  the  showing  of  a  ghost  is  concerned,  I  can  do  that 
here  myself  for  you  in  this  very  room." 

"  But  you  can't  do  it  as  they  do  it  at  the  Polytechnic," 
said  John. 

"  On  the  same  principle,"  said  Lawrence.  "  I  could  not 
do  it  on  so  large  a  scale,  and  I  could  not  make  the  illusion 
complete,  because  I  could  not  conceal  from  you  the  means 
by  which  I  should  do  it ;  but  I  should  do  it  in  pi'ecisely  the 
same  way,  so  far  as  the  optical  principles  are  concerned. 
Still,  I  should  like  very  much  to  see  how  they  do  it  at  the 
Polytechnic." 

"  And  I  should  like  to  see  how  you  do  it  your  way,"  said 
John. 

"  Very  well,"  said  LaAvrence. 

So  saying,  Lawrence  took  from  the  table  a  card,  and  cut 
out  from  it  a  figure  which  formed  a  rude  resemblance  to  a 
ghost — that  is,  a  fantastic  figure,  with  its  arms  extended, 
and  covered  with  a  sheet. 

There  was  a  shelf  outside  the  window  of  the  room  that 
Lawrence  and  John  were  in,  placed  there  apparently  to 
hold  flower-pots,  though  there  were  at  that  time  no  flower- 
pots upon  it.  There  were  also  upon  the  mantel-piece  a 


THE    CAT   AND    THE    GHOST.  99 

number  of  ornaments,  and  among  the  rest  an  image  of  a 
cat,  in  a  sitting  posture.  Lawrence  went  to  the  mantel- 
piece and  took  this  image  from  it,  and  then  opened  the 
window  and  set  the  image  outside  upon  the  edge  of  the 
shelf. 

"  There,"  said  he, "  I  am  going  to  see  if  I  can't  frighten 
that  cat  with  a  ghost." 

He  then  lighted  one  of  the  candles  by  means  of  a  match, 
and  brought  it  near  to  the  window,  holding  his  ghost  in 
one  hand  and  the  candle  in  the  other.  He  placed  John 
near  the  window,  directing  him  to  look  at  the  cat.  Then 
he  held  the  figure  of  the  ghost  a  little  behind  John,  and 
over  his  head,  and  also  held  the  candle  in  such  a  position 
that  it  should  shine  upon  the  figure,  and  yet  not  shine  into 
John's  eyes.  The  consequence  was  that  a  strong  light  was 
thrown  upon  the  paper  ghost,  which  made  the  reflection 
of  it  plainly  visible  near  the  place  where  the  cat  was  seen 
by  direct  vision.  In  other  words,  the  light  from  the  image 
of  the  cat  came  through  the  glass  from  the  outside  by 
transmission,  while  that  from  the  ghost,  from  the  inside,  on 
striking  the  glass,  came  back  to  the  eye  by  reflection  ;  and 
thus  the  two  images  were  produced  side  by  side  on  the  ret- 
ina of  the  eye. 

"  Is  that  the  way  they  do  it  ?"  asked  John. 

"Yes,"  said  Lawrence, " that  is  exactly  the  way  in  prin- 
ciple, only  they  do  it  on  a  much  larger  scale,  and  they  con- 
trive to  conceal  the  means.  Here,  for  instance,  the  ghost, 
being  only  a  paper  card,  is  lifeless,  and  the  cat  is  lifeless 
too ;  whereas  the  effect  would  be  the  same  if  they  were 
both  alive,  and  could  move,  so  as  to  increase  the  appear- 
ance of  reality." 

"  A  cat  would  not  do  for  that,"  said  John, "  for  cats  are 
not  afraid  of  ghosts." 

"How  do  you  know?"  asked  Lawrence. 


100  SPECTRES   AND   GHOSTS. 

"  Because,  if  they  were,"  said  John, "  they  would  not  dare 
to  go  about  nights  as  much  as  they  do." 

"  They  could  have  a  man,  then,"  said  Lawrence, "  and  he 
could  pretend  to  shoot  the  ghost,  which  it  would  be  very 
safe  for  him  to  do,  since  he  would  only  shoot  at  nothing — 
in  the  air.  The  real  person  by  which  the  appearance  was 
produced  would  be  on  the  other  side  of  the  glass,  far 
away." 

"  He  might  break  the  glass,  then,"  said  John. 

"  No,"  replied  Lawrence, "  for  the  image  is  not  in  the 
glass,  but  far  behind  it — as  far  behind  it,  in  fact,  as  the  real 
object  is  before  it.  The  image  of  my  little  ghost,  for  in- 
stance, appears,  not  on  the  surface  of  the  window-pane,  but 
out  on  the  shelf,  where  the  cat  is — as  far,  in  fact,  from  the 
glass  on  the  outside  as  I  hold  the  paper  from  it  on  the  in- 
side." 

Here  Lawrence  moved  his  card  backward  and  forward, 
nearer  to  and  farther  from  the  glass,  and  showed  that  the 
image  seemed  to  advance  and  recede  in  a  manner  exactly 
corresponding  to  the  movement  of  the  object. 

Any  i-eader  of  this  book  can  see  how  this  was  done  by 
cutting  out  such  an  image  and  holding  it  up  near  the  win- 
dow, with  a  lamp  or  candle  near,  to  illuminate  it  strongly. 
The  effect  will  be  greater  if  this  is  done  in  the  evening,  be- 
fore it  is  quite  dark,  so  that  there  shall  be  no  bright  light 
shining  upon  the  objects  in  the  street,  but  only  enough  to 
make  them  visible. 

"If  I  wished  to  make  a  representation  of  the  ghost  at 
the  other  side  of  the  street,"  continued  Lawrence, "  then  I 
should  have  to  carry  back  my  paper  ghost  to  the  back  side 
of  the  room,  and  make  it  a  great  deal  larger.  I  might  have 
a  real  person,  with  a  sheet  over  his  arms,  to  represent  the 
ghost,  so  that  he  might  make  gestures  and  walk  about,  but 
the  principle  would  be  the  same  in  that  case  as  in  this. 


THE    ENGRAVING   EXPLAINED.  103 

But  even  then  the  illusion  would  not  be  complete,  for  the 
real  ghost,  and  the  light  shining  upon  him,  and  also  the 
glass  of  the  window  in  which  he  was  reflected,  would  all 
be  seen.  In  the  contrivances  for  producing  these  illusions 
on  the  stage,  all  these  things  are  concealed." 

So  saying,  Lawrence  took  a  book  out  of  his  trunk  and 
opened  to  an  engraving  in  which  the  arrangements  usually 
made  for  producing  such  illusions  on  the  stage  were  repre- 
sented and  fully  explained. 

You  see  here  a  copy  of  this  engraving.  The  glass  is  a 
lai-ge  piece  of  plate-glass,  like  a  very  large  looking-glass 
without  any  silvering  on  the  back,  and  it  covers  the  whole 
front  of  the  stage.  In  the  engraving  the  edges  of  the  glass 
are  seen,  but  in  the  actual  performance  these  edges  are 
concealed  by  the  curtains  coming  close  to  them  on  each 
side.  Of  course  the  glass  must  be  extremely  large.  It  is 
placed,  too,  in  an  inclined  position,,  so  that  all  the  light 
shining  upon  it  from  the  side  toward  the  audience  is  re- 
flected downward,  and  thus  it  produces  no  "glare." 

The  figure  which  is  to  form  the  image  of  the  ghost  in 
the  glass  is  in  front  of  the  stage,  and  is  placed  low  enough 
to  be  entirely  concealed  from  the  view  of  the  spectators. 
It  is  strongly  illuminated  by  a  very  bright  light,  which  is 
also  concealed  under  the  stage.  The  light,  of  course,  radi- 
ates in  every  direcfion  from  the  figure,  and  spreads  over 
the  whole  surface  of  the  glass,  whence  it  is  reflected  all 
over  the  theatre ;  and  a  certain  portion  of  it,  sufficient  to 
form  an  image  upon  the  retina,  reaches  every  spectator, 
and  such  an  image,  moreover,  as  if  the  light,  instead  of 
coming  up  from  beneath  the  stage,  and  being  reflected  by 
the  glass,  had  really  come  from  an  equal  distance  behind 
the  stage — that  is,  from  the  place  where  the  image  seems 
to  stand,  near  the  man  who  is  pointing  a  pistol  at  it. 

This  man  would,  however,  not  see  any  image  at  all  near 


104  SPECTRES   AND   GHOSTS. 

him,  any  more  than  a  person  behind  a  looking-glass  stand- 
ing out  upon  a  floor  would  see  the  images  reflected  in  it  to 
those  before  it.  The  figure  is  represented  there  only  to 
show  how  it  wrould  appear  to  the  people  in  front. 

In  the  same  manner,  the  outline  of  the  figure  upon  the 
glass  itself  is  only  imaginary,  being  sketched  there  only  to 
show  how  and  where  the  light  is  reflected.  There  would 
be  no  such  image  really  there.  The  only  things  that  would 
actually  exist  would  be  the  figure  below,  the  light  emana- 
ting from  it,  and  the  images  of  it  in  the  eyes  of  the  specta- 
tor. In  other  words,  what  the  spectators  really  see  is  the 
figure  itself,  only  they  see  it  by  light  coming  to  them  in 
zigzag  lines,  or,  at  least,  lines  taking  one  sharp  turn,  as  it 
is  reflected  in  the  glass,  instead  of  in  direct  lines ;  and  as 
the  short  turn  taken  by  the  rays  of  light  at  the  glass  is  al- 
together outside  of  the  eye,  their  sense  takes  no  cognizance 
of  it,  and  the  object  appears  to  them  as  if  it  were  seen  di- 
rectly before  them  on  the.  stage. 


A  VISIT  TO   THE   POLYTECHNIC.  105 


CHAPTER  XH. 

THE   POLYTECHNIC  INSTITUTION. 

LAWKENCE  and  John  set  off  one  evening  about  seven 
o'clock  to  go  to  the  Polytechnic.  They  went  up  through 
Regent  Street,  and,  as  they  had  plenty  of  time  before  them, 
they  stopped  along  the  way  to  look  into  tl  e  shop-windows, 
to  examine  and  admire  the  curious  and  beautiful  things 
that  were  to  be  seen  in  them.  At  length  they  arrived  at 
their  destination.  The  building  presented  somewhat  the 
appearance  of  a  church.  They  passed  in  through  the 
porch  in  company  with  many  other  persons,  and  stopped 
on  one  side,  as  they  went  in,  at  a  little  office  to  buy  their 
tickets.  The  tickets  were  a  shilling — that  is,  an  English 
shilling — each.  The  value  of  an  English  shilling  is  about 
a  quarter  of  a  dollar. 

When  they  were  fairly  within,  they  found  that  the  inte- 
rior of  the  principal  room  was  still  more  similar,  in  its  ar- 
rangements, to  a  church  than  the  exterior  had  been,  for  it 
was  of  an  oblong  shape,  with  an  open  floor  below,  and  gal- 
leries above  all  around  supported  by  pillars ;  only,  instead 
of  pews,  the  floor  below  was  filled  with  small  steam-engines 
and  large  articles  of  apparatus ;  and  the  floor  of  the  gal- 
lery was  also  flat,  and  had  a  range  of  curiosities  and  little 
machines  over  the  balustrade  in  front,  and  upon  shelves 
against  the  wall  behind,  with  an  open  space  for  people  to 
walk  in  between.  Where  the  pulpit  usually  is  in  a  church 
there  was  a  great  tank,  ten  feet  square  and  very  deep,  which 
was  filled  with  water,  and  over  it  there  was  hanging  an 
immense  diving-bell,  ready  to  go  down. 
E2 


106  THE   POLYTECHNIC   INSTITUTION. 

Lawrence  and  John  walked  about  for  some  time  among 
these  things,  observing  and  examining  all  that  they  saw, 
until  at  length  they  came  back  to  what  would  be  called  in 
a  church  the  singers'  gallery,  where  there  was  a  seat,  upon 
which  they  could  sit  and  rest  themselves,  and,  at  the  same 
time,  look  down  over  the  balustrade  to  the  scene  below. 

Immediately  beneath  them,  on  the  lower  floor,  was  a 
steam-engine,  and  various  other  machines  connected  with 
it.  A  little  farther  on  was  a  large  table  with  a  blowpipe, 
and  several  other  contrivances  attached  to  it  for  glass- 
blowing.  The  glass-blower  was  sitting  at  the  table  at 
work,  making  a  great  many  curious  things,  and  a  number 
of  persons — young  men,  young  women,  and  children — were 
standing  around  the  table  watching  the  operations,  and 
buying  the  articles  which  the  man  made  as  soon  as  fin- 
ished, or  selecting  them  from  a  large  supply  of  similar  ar- 
ticles which  he  had  on  the  table  before  him,  and  which  he 
had  previously  made. 

It  was  curious,  John  thought,  to  see  the  process  of  spin- 
ning glass  silk  as  this  man  performed  it.  He  had  on  his 
left  hand  a  wheel,  about  a  foot  in  diameter,  which  was 
mounted  on  a  stand,  and  made  to  turn  by  a  crank.  To 
spin  the  glass  thread,  or  silk,  he  held  the  end  of  a  glass  rod 
in  the  flame  of  the  blowpipe  before  him,  to  keep  a  portion 
of  it  melted,  or  at  least  softened  enough  to  be  drawn  out 
into  a  fine  thread,  and  this  thread,  as  fast  as  it  was  drawn, 
he  wound  round  the  wheel,  turning  the  wheel  all  the  time 
with  his  left  hand. 

Of  course  Lawrence  and  John  could  not  see  the  thread 
from  the  distance  at  which  they  were  sitting,  it  was  so  fine ; 
but  they  could  see  the  glass  rod,  and  the  glow  of  light  at 
the  end  of  it,  where  it  touched  the  point  of  the  flame  of  the 
blowpipe,  and  they  could  also  see  the  wheel  turn,  which 
served  as  a  reel  to  wind  the  skein  upon. 


GLASS   SPUN   INTO   THREADS.  107 

"  I  should  not  have  thought  it  would  be  possible  to  spin 
glass  in  that  way,"  said  John,  "  and  certainly  not  to  spin 
it  so  even  and  true.  I  looked  at  a  piece  of  it  when  I  was 
down  there,  and  it  was  just  as  even  in  every  part  as  a  hair, 
and  very  fine,  and  yet  the  man  does  not  seern  to  take  any 
pains  to  make  it  so  even." 

"  It  evens  itself,"  said  Lawrence.  "  There  are  one  or 
two  very  curious  principles  involved.  There  is  one  that 
you  will  think  is  specially  curious,  if  I  can  only  explain  it 
so  that  you  will  understand  it.  You  see  that,  the  hotter 
the  glass  is,  the  weaker  it  is,  because  it  is  more  fluid ;  while 
the  colder  it  is,  the  stronger  and  stiffer  it  is.  Now,  in  draw- 
ing out  the  glass,  it  gets  cold  and  strong  just  in  proportion 
as  it  gets  thin,  and  that  holds  it  in  the  thin  and  slender 
places,  and  prevents  it  getting  thinner. 

"In  other  words,"  continued  Lawrence,  "just  so  fast  as 
any  parts  of  the  thread  get  thinner  than  the  rest,  they  cool 
at  once,  and  become  stronger,  and  that  brings  the  force  to 
act  upon  the  parts  which  are  thicker,  and,  of  course,  more 
soft,  and  they  are  drawn  out  until  they  are  as  thin  as  the 
rest." 

The  same  principle  operates  on  a  larger  scale  in  pulling 
candy,  and  it  is  by  the  effect  of  it  that  it  is  so  easy  to  make 
the  candy  into  sticks  so  uniform  and  even. 

Just  at  this  moment  a  bell  was  rung,  and  there  was  ah 
immediate  movement  among  the  audience  toward  a  certain 
door  behind  the  place  where  Lawrence  and  John  had  been 
sitting.  They  immediately  rose  and  followed  the  multi- 
tude, supposing  that  the  bell  was  a  summons  to  hear  the 
lecture  on  ghosts.  There  was  a  great  crowding  and  jam- 
ming in  getting  through  the  door,  each  one  apparently  be- 
ing eager  to  obtain  a  good  seat. 

Lawrence  and  John  pressed  onward  with  the  crowd, 
and,  when  they  at  last  entered  the  room,  they  found  that 


108  THE    POLYTECHNIC   INSTITUTION. 

it  was  a  lecture-room  arranged  somewhat  like  a  theatre. 
There  was  a  small  stage  at  the  farther  side,  with  a  curtain 
drawn  before  it.  In  front  of  the  stage  was  a  narrow  area, 
which  was  open,  and  a  desk,  or  small  pulpit,  with  a  table 
by  the  side  of  it,  for  the  lecturer,  at  one  end,  and  something 
that  looked  like  a  large  and  tall  box  on  wheels  at  the  other 
end.  Immediately  back  of  this  open  area  were  the  seats 
for  the  audience,  wThich  rose  one  above  another  by  a  rapid 
ascent,  so  that  every  body  could  see. 

There  were  various  exhibitions  and  performances  during 
the  lecture,  which  continued  for  about  half  an  hour.  The 
lecturer  was  the  celebrated  Professor  Pepper,  who  is  dis- 
tinguished for  his  tact  and  skill  in  explaining  and  elucida- 
ting philosophical  principles,  and  making  every  thing  clear. 
He  had  an  assistant  with  him,  and  the  first  thing  to  be  done 
was  to  darken  the  room,  and  then  throw  a  beam  of  very 
strong  light  from  a  kind  of  lantern  that  the  assistant  had 
upon  the  table  across  the  area — or,  rather,  along  the  area 
from  one  side  of  the  room  to  the  other — before  the  specta- 
tors. The  beam  made  a  round  and  very  bright  spot  upon 
the  wall,  but  was  not  visible  on  its  way  through  the  air, 
or  scarcely  visible,  because  there  was  nothing  there  to  in- 
tercept and  reflect  it  to  the  eyes  of  the  spectators.  For 
you  will  recollect  that,  as  has  been  explained  before,  light 
is  not  visible  by  itself  as  it  passes  through  space  before  us, 
but  only  so  far  as  it  is  intercepted  by  some  substance  and 
turned  from  its  course,  and  so  directed  into  the  eye. 

There  were,  however,  some  few  minute  motes  and  parti- 
cles of  dust  floating  in  the  air  of  the  room  over  the  area, 
which  served  this  purpose  of  intercepting  and  reflecting 
the  light  in  some  slight  degree,  so  that  the  path  of  the 
beam  through  the  air  was  not  wholly  invisible.  The  as- 
sistant, however,  soon  brought  it  very  clearly  into  view  by 
using  something  which  emitted  a  thick  smoke  or  dust  of 


EXHIBITION    OF   THE    GHOSTS.  109 

some  kind,  and  which  he  beat  in  the  path  of  the  beam  of 
light  so  as  to  make  it  very  distinctly  and  beautifully  visi- 
ble. The  professor  then  held  mirrors  and  lenses  of  various 
kinds  in  the  path  of  the  beam,  so  as  to  turn  the  light  in  va- 
rious directions,  and  change  the  condition  of  it  in  various 
ways,  thus  causing  it  sometimes  to  converge  and  come  to 
a  point,  and  sometimes  to  diverge  and  diffuse  itself,  in 
which  case  it  made  a  very  large  and  bright  spot  upon  the 
ceiling  overhead,  or  upon  any  part  of  the  wall  on  which  he 
caused  it  to  fall.  The  assistant  all  the  time  continued  to 
puff  the  smoke  or  dust  into  the  path  of  light,  so  as  to  make 
its  course  distinctly  visible. 

After  some  farther  experiments  and  illustrations  of  this 
kind,  the  time  came  for  the  ghosts.  The  curtain  rose  and 
brought  to  view  a  small  stage,  like  that  of  a  theatre,  only 
the  front  of  it  was  closed  by  an  immense  pane  of  plate- 
glass,  which  must  have  been  some  ten  feet  square.  This 
glass  was,  however,  not  at  all  noticed  by  the  audience,  for 
it  was  inclined  forward  at  such  an  angle  as  to  throw  the 
reflections  of  all  the  light  that  came  from  the  side  toward 
the  audience  down  toward  the  floor,  and  under  the  front 
of  the  stage,  so  that  none  except  those  who  were  in  the 
secret  had  any  idea  that  there  was  any  glass  there.  The 
edges  of  it,  at  the  ends,  were  well  concealed  by  curtains 
coming  up  close  to  it. 

The  spectators,  therefore,  did  not  see  either  the  glass 
itself,  or  any  thing  reflected  in  it.  Their  reflections  were 
all  thrown  downward.  It  is  true  that  if  there  had  been 
any  thing  bright  down  beneath  the  front  of  the  stage  they 
would  have  seen  the  reflection  of  it  coming  up  to  them ; 
bnt  good  care  had  been  taken  to  prevent  that  by  making 
it  dark  there.  The  ghost  was  there,  or,  rather,  the  person 
who  was  to  represent  the  ghost,  but  there  was  no  light  yet 
shining  upon  him  to  be  reflected  by  the  glass  toward  the 


HO  THE    POLYTECHNIC    INSTITUTION. 

audience,  and  so  the  audience  saw  nothing  in  the  glass, 
but  only  saw  through  it,  and,  of  course,  only  saw  what  was 
actually  upon  the  stage  before  them. 

Things  being  thus  arranged,  all  that  was  necessary  was 
to  allow  the  real  person  on  the  stage  to  talk  and  act  as 
usual  until  the  time  came  for  the  ghosts  or  hobgoblins  to 
appear,  when  all  at  once  a  very  bright  light  was  thrown 
upon  the  objects  representing  these  things  under  the  stage, 
when  all  the  spectators  in  the  seats  would  see  them  re- 
flected in  the  glass ;  and,  as  the  images  of  them  would 
appear  as  far  behind  the  glass  as  the  objects  themselves 
were  before  it,  they  would  seem  to  be  back  upon  the 
stage,  among  the  real  actors. 

There  is  one  curious  difficulty,  however,  in  the  manage- 
ment of  such  an  exhibition,  and  that  is,  that  without  some 
special  contrivance  to  prevent  the  effect,  the  position  of  the 
mirror,  inclined  at  an  angle  of  forty-five  degrees  more  or 
less,  would  have  the  effect  of  making  the  floor  on  which 
the  personations  of  ghosts  and  goblins  stood  under  the 
front  of  the  stage  appear  in  the  glass  in  a  perpendicular 
position — that  is,  up  and  down — so  that  the  images  in  it 
would  appear  standing  out  upon  the  wall,  in  an  impossible 
attitude.  John  had  observed  in  the  advertisement  of  the 
exhibition  in  the  papers  that  the  ghosts  would  "  dance  on 
walls  and  ceilings,"  and  he  had  at  first  imagined  that  the 
being  able  to  make  them  do  so  would  be  the  special  won- 
der of  the  performance,  and  would  require  very  particular 
and  extraordinary,  and  even,  perhaps,  quite  complicated 
optical  arrangements,  instead  of  being,  as  it  really  is,  a 
very  difficult  thing  to  avoid. 

Any  one  can  see  this  for  himself  by  means  of  any  look- 
ing-glass— a  small  one  will  answer  the  purpose  perfectly 
well.  You  place  this  glass  on  a  table  before  you,  first  hold- 
ing it  in  an  upright  position.  You  place  any  object  before 


EXPERIMENT   WITH   A   MIRROR.  113 

it,  a  small  doll,  for  example.  Now,  so  long  as  the  glass 
and  the  doll  are  both  upright,  the  image  of  the  doll  in  the 
glass  will  appear  upright,  and  the  table,  as  reflected  in  the 
glass,  will  appear  level,  as  it  is  in  reality.  But  the  moment 
that  you  begin  to  tip  the  glass  forward,  the  reflected  por- 
tion of  the  table  will  begin  to  rise  up,  and  the  reflected 
image  of  the  doll  will  incline  forward,  and  what  at  first 
thought  seems  singular,  the  apparent  movement  of  what  is 
seen  in  the  mirror  upward  and  forward  will  be  twice  as 
great  as  that  of  the  mirror  itself  forward  and  downward; 
so  that  when  the  mirror  is  inclined  at  an  angle  of  forty- 
five  degrees — that  is,  half  way  down  to  the  table — the  re 
fleeted  part  of  the  table  will  be  perpendicular,  and  the  doll, 
instead  of  standing  upright,  will  be  projected  forward,  as 
if  she  were  standing  on  a  wall. 

A  very  good  thing  to  try  this  experiment  with,  especial- 
ly when  older  brothers  or  sisters  wish  to  show  it  to  the 
younger  children,  is  an  image  of  a  mouse,  and  then  the 
mouse  will  seem,  when  reflected,  as  if  running  up  or  com- 
ing down  a  wall. 

Thus,  instead  of  there  being  any  difficulty  in  represent- 
ing the  ghosts  and  goblins  as  appearing  to  be  on  a  wall, 
the  real  difficulty  is  to  make  them  appear  to  be  on  a  level 
floor. 

There  are  various  means  and  contrivances  used  to  ac- 
complish this  last  purpose,  one  of  which  is  to  have  another 
glass,  to  reflect  the  light  a  second  time,  and  so  bring  the 
position  right. 

Another  is  to  have  the  person  representing  the  ghost,  or 
the  figure,  whatever  it  is,  placed  horizontally  on  the  floor, 
and  thus  it  will  appear,  when  reflected,  as  if  standing  back 
against  the  wall.  You  can  obtain  a  general  idea  how  this 
is  done  by  holding  the  looking-glass  in  an  inclined  position 
before  you  on  the  table,  and  then  placing  the  doll  on  its 


114  THE   POLYTECHNIC   INSTITUTION. 

back  on  the  table,  with  its  feet  toward  the  glass.     The  doll 
will  thus  appear  in  an  upright  position  in  the  glass. 

In  these  expei-iments  which  you  make  with  the  glass,  a 
common  table  will  be  found  too  low,  except  for  young 
children  whose  heads  just  come  up  above  the  level  of  the 
upper  surface  of  it ;  for  the  head  of  the  spectator  ought  to 
be  about  on  a  level  with  the  middle  of  the  glass.  A  chair 
placed  upon  a  table — a  kitchen  table,  for  example — will 
bring  the  glass,  perhaps,  at  about  the  right  height  for 
young  persons  from  twelve  to  fifteen  years  of  age. 

In  the  exhibition  which  Lawrence  and  John  witnessed 
at  the  Polytechnic  there  were  several  different  perform- 
ances, in  which  quite  a  number  and  variety  of  phantasms 
were  made  to  appear.  One  was  the  figure  of  a  statue, 
which  had  the  appearance  of  standing  back  against  the 
wall  of  a  painter's  studio.  Of  course  it  was  produced  by 
some  kind  of  statue  in  pasteboard,  which  was  lying  in  a 
horizontal  position  beneath  the  front  of  the  stage.  It  could 
be  made  to  appear  and  disappear  at  pleasure  by  throwing 
a  strong  light  upon  it  or  shutting  the  light  off.  Then 
there  were  figures  also — some  that  represented  hobgoblins 
that  ran  about  upon  the  wall.  One  was  in  the  form  of  a 
monstrous  fat  lizard,  with  four  paws  and  a  long  tail,  that 
crawled  about  in  a  most  mysterious  manner  as  he  appeared " 
reflected  in  the  glass.  Of  course  this  animal  was  really  a 
boy,  with  an  artificial  shell  or  coat  to  represent  an  uncouth 
green  monster. 

Then  there  were  a  number  of  very  pretty  and  agile  lit- 
tle fairies  in  gorgeous  dresses,  that  danced  about  in  the 
most  fantastic  manner,  so  much  so  that  it  was  difficult  to 
follow  them,  and  to  tell  whether  they  were  upon  the  wall, 
in  the  air,  or  upon  the  floor. 

After  the  exhibition  had  been  continued  for  some  min- 
utes, and  it  was  time  to  bring  the  lecture  to  a  close,  Pro- 


THE    LITTLE    FAIRIES.  115 

fessor  Pepper  caused  a  small  curtain  in  front  of  and  below 
the  stage  to  be  lifted  up,  so  that  by  looking  down  the  spec- 
tators could  see  the  objects  directly  that  they  had  before 
seen  reflected  in  the  glass  above — the  pasteboard  statue, 
the  lizard,  and  the  other  similar  monsters,  and,  what  was 
prettier  than  all,  the  little  fairies,  who  proved  to  be  young 
and  agile  girls,  dressed  gayly  as  dancers.  There  was  a 
very  bright  light  shining  upon  them,  and  the  girls  bowed 
and  smiled,  and  made  salutations  to  the  audience  in  a 
charming  manner.  A  moment  afterward  the  light  was 
suddenly  shut  off,  the  fairies  and  hobgoblins  all  vanished 
in  an  instant,  the  curtain  which  had  concealed  them  was 
dropped,  the  gas  was  turned  on  above  so  as  to  brighten  up 
the  whole  room,  and  the  performance  was  over. 


116  VERY   HEIGHT  LIGHTS. 


CHAPTER 

VERY    BRIGHT    LIGHTS. 

FOR  such  performances  as  those  which  Lawrence  and 
John  witnessed  at  the  Polytechnic,  and  also  for  many  oth- 
er purposes,  a  very  bright  light  is  required.  There  are 
modes  of  producing  artificial  light  of  such  intense  bril- 
liancy that  you  can  not  look  upon  it  directly  with  the 
naked  eye  for  a  moment. 

But,  though  we  can  not  look  upon  the  light  itself  with- 
out dazzling  the  eye,  the  illumination  which  it  produces 
when  shining  upon  other  objects,  though  exceedingly 
bright,  is  very  beautiful  to  see.  Then,  moreover,  when 
objects  are  to  be  seen  by  reflection  in  a  glass,  there  is  great 
advantage  in  being  able  to  illuminate  them  by  a  light  so 
strong  that  it  can  not  be  viewed  directly  without  dazzling 
the  eyes. 

Besides  this,  there  are  a  great  many  cases  in  which  light 
is  diminished  by  diffusion  instead  of  by  reflection,  and  as 
the  diffusion  weakens  it,  as  has  already  been  explained,  in 
the  ratio,  for  similar  purposes,  of  the  squares  of  correspond- 
ing lines,  the  light  must  be  very  bright  indeed  at  the 
source,  in  order  that  it  may  be  bright  enough  after  diffu- 
sion. 

The  engraving  on  the  opposite  page,  for  example,  repre- 
sents what  is  called  a  magic  lantern.  It  consists  of  a  kind 
of  lantern,  with  an  apparatus  within  it  capable  of  produc- 
ing an  intense  light,  and  also  of  concentrating  this  light  at 
a  point,  from  which  it  afterward  diverges  in  such  a  manner 
as  to  produce  an  enlarged  shadow,  or  image,  of  any  minute 


LIGHT-HOUSES. 


117 


TUB  MAQIO  I.ANTEBN. 


object  placed  near  the  focus  of  the  light,  and  throwing  it 
upon  a  screen  across  the  room,  where  many  persons  can 
see  it  together. 

The  first  purpose,  however,  for  which  the  need  of  a  very 
bright  light  was  felt  by  mankind  was  to  increase  the  range 
of  illumination  spread  over  the  sea  from  the  lanterns  in 
light-houses.  Light-houses  on  sea-coasts  have  been  in  use 
from  very  ancient  times.  It  is  true  that,  before  the  inven- 
tion of  the  mariner's  compass,  ships  were  very  seldom  taken 
intentionally  far  out  of  sight  of  land.  But  they  were  at  any 
time  liable  to  be  driven  off  the  coast  by  sudden  storms,  or 
to  have  shores  that  were  near  hidden  from  view  by  mists, 
or  fogs,  or  driving  rain ;  and  sometimes  their  voyage  would 
be  protracted  by  unfavorable  winds,  so  that  night  would 
come  on  before  they  had  entered  their  destined  port. 


118 


VERY    BRIGHT   LIGHTS. 


From  the  effect  of  these  and  other  similar  causes,  lights 
placed  at  certain  points  along  frequented  coasts  were  very 
eai'ly  used,  long  before  any  means  were  known  of  produc- 
ing any  light  brighter  than  that  afforded  by  an  ordinary 
fire,  or,  rather,  from  such  as  could  be  produced  by  the  most 
combustible  natural  substances  that  could  be  obtained, 
such  as  resinous  wood,  or  porous  materials  saturated  with 
pitch,  or  bitumen,  or  oil.  These  substances  were  placed 
sometimes  in  an  iron  receptacle  called  a  cresset,  which  was 
raised  upon  the  summit  of  a  high  tower,  the  system  requir- 
ing, of  course,  the  constant  attendance  of  a  guardian  to 
watch  and  continually  replenish  the  fire. 

The  vessel  containing  the  fire  was  called  a  cresset,  from 
the  word  croisette,  a  little  cross, 
the  iron-work  being  often  sur- 
mounted with  a  cross,  in  token 
of  the  dependence  of  the  poor 
mariners,  in  their  exposures  to 
the  terrible  dangers  of  the  sea, 
on  the  special  protection  of 
heaven. 

There  is  one  thing  which  it  is 
very  important  to  observe  in 
respect  to  the  manner  in  which 
lights  upon  a  sea-coast  aid  the 
mariner  in  finding  his  way  over 
the  dark  waters,  and  that  is,  that 
the  object  is  entirely  different 
from  that  of  light  in  other  cases, 
as,  for  instance,  those  in  the  street,  or  in  a  room.  These  last 
are  intended  to  illuminate  the  surrounding  objects  so  that 
they  can  be  seen.  In  these  and  in  all  other  ordinary  cases, 
the  use  of  the  light  is  not  to  make  itself  seen,  but  to  illu- 
minate the  objects  that  it  shines  upon  so  that  they  can  ba 


ANCIENT   LIGHT-HOUSE. 


OBJECT   OP   THE    LIGHT-HOUSE.  119 

seen.  But  the  purpose  of  the  light  from  a  light-house  is 
not  to  enable  the  observer  to  see  any  thing  except  itself, 
but  to  see  itself  only  for  the  purpose  of  enabling  him  to 
determine  where  he  is.  It  does  not  make  visible  to  him 
the  entrance  into  the  harbor,  nor  show  him  the  rocks  and 
shoals  which  he  is  to  avoid,  but  only  to  show  itself,  and,  by 
so  doing,  to  mark  a  point,  for  the  purpose  simply  of  mak- 
ing known  to  the  mariner  where  he  is.  Being  guided  in 
this  way  only  in  determining  his  position,  he  must  depend 
upon  his  chart,  or  his  own  knowledge  of  the  coast  lines 
near,  for  his  guidance  into  the  entrance  of  his  haven. 

Thus  it  happens  that,  for  a  beacon  on  the  shore  of  the 
sea,  there  is  required,  not  a  diffused,  but  a  highly  concen- 
trated light,  to  show  itself  to  the  mariner  simply  as  a  star 
beaming  from  the  midst  of  surrounding  darkness.  All  that 
the  mariner  requires  of  it  is  that  it  should  show  itself  to 
him.  He  does  not  expect  that  it  will  reveal  to  him  any 
of  the  surrounding  objects.  These  he  must  have  in  his 
memory,  or  in  the  mental  conceptions  which  he  forms  from 
his  chart.  The  light  is  only  to  enable  him  to  place  himself 
properly  among  them. 

There  is  another  thing  that  is  remarkable  and  is  very 
important  to  be  understood  in  respect  to  such  a  light,  and 
that  is,  that  it  is  only  that  portion  of  it  which  shines  in 
certain  limited  directions  that  is  useful  for  the  purpose  re- 
quired— namely,  that  which  goes  forward  over  the  sea — 
and  of  this  only  that  portion  which  passes  along  at  a  mod- 
erate distance  above  the  surface  of  the  water.  The  light 
from  any  luminous  point  radiates  naturally,  as  has  already 
been  explained,  in  every  direction,  so  as  to  illuminate  a 
complete  sphere.  A  very  large  portion  of  this  sphere  is 
cut  off,  of  course,  by  the  ground.  Half  of  it  would  be  so 
cut  off  if  the  light  was  at  the  surface  of  the  ground  and 
the  ground  was  level ;  but,  as  the  light  is  raised  above  the 


120  VEKY    BRIGHT   LIGHTS. 

surface,  an  amount  less  than  one  half,  though  still  a  very 
large  portion,  is  thus  intercepted. 

Then,  moreover,  as  the  light  of  a  light-house  is  not  in- 
tended to  guide  travelers  by  land,  all  that  would  naturally 
shine  on  the  landward  side,  if  it  were  allowed  to  have  its 
own  way,  would  be  entirely  lost.  In  the  same  manner,  as 
it  is  not  intended  to  shine  for  the  benefit  of  the  birds  in 
the  air,  all  that  would  go  upward  would  be  lost.  In  a  word, 
it  is  only  that  comparatively  small  portion  of  the  sphere  of 
radiance  that  extends  forward  over  the  surface  of  the  sea, 
and  at  a  small  distance  above  it — as  high  as  the  deck  of 
any  vessel — that  is  of  any  use  for  the  purpose  designed. 

Now  in  ancient  times,  when  these  lights  consisted  sim- 
ply of  blazing  fires  on  the  summit  of  a  tower,  all  except 
this  small  portion  was  lost;  but  in  modern  times  means 
have  been  found  to  avoid  this  loss  by  bending  that  portion 
of  the  rays  that  would  naturally  take  a  wrong  direction 
into  the  right  one — that  is,  by  intercepting  all  or  nearly 
all  those  rays  which  would  go  back  over  the  land,  or  down 
into  the  ground,  or  up  into  the  air,  and  turning  them  in  the 
direction  where  their  services  are  required  —  that  is,  out 
over  the  water.  This  is  done  by  certain  extremely  inge- 
nious contrivances,  through  the  effect  of  which  the  rays 
which  issue  from  the  source  of  light  are  collected  on  all 
sides  and  made  to  shoot  forward  over  the  sea,  so  that,  in- 
stead of  forming  a  sphere,  the  range  of  illumination  takes 
the  form  of  a  flat  wheel,  or,  rather,  half  a  wheel,  extending 
forward  over  the  water,  and  lying  very  low. 

And  inasmuch  as  we  can  only  see  any  object  when  the 
rays  from  it  enter  the  eye,  we  can  only  see  the  light  from 
a  light-house  when  we  are  placed  within  this  range.  Thus 
people  on  the  land  behind  a  light-house  would  not  see  the 
light  of  it  at  all,  nor  would  birds  in  the  air.  A  bird  that 
had  alighted  on  the  mast-head  of  a  ship  coming  in  a  dark 


STAGE-LIGHTS.  121 

night  toward  the  coast  would  see  the  light  of  the  light- 
house like  a  very  brilliant  star  in  the  horizon ;  but  if  she 
should  leave  her  perch  and  fly  a  few  hundred  feet  into  the 
air,  she  would  lose  sight  of  it,  and  she  might  well  wonder 
what  had  become  of  it.  The  truth  would  be,  that  all  the 
light  which  would  naturally  have  come  to  that  point 
would  have  been  bent  downward  near  to  the  surface  of 
the  sea,  for  the  benefit  of  the  mariners  on  the  decks  of  their 
vessels,  leaving  the  regions  of  the  upper  air  in  darkness, 
the  illumination  not  being  intended  for  the  benefit  of  the 
birds. 

There  is  required,  of  course,  a  very  bright  and  concen- 
trated light  for  such  purposes  as  this,  in  order  that  the 
necessary  amount  of  illumination  may  be  brought  within 
such  a  compass  that  the  apparatus  within  which  it  is  con- 
tained, and  the  lenses  and  reflectors  required  for  throwing 
all  the  radiation  from  it  out  over  the  sea,  may  not  be  of  an 
inconvenient  or  unmanageable  size. 

A  very  bright  light  is  also  required  for  the  spectral  illu- 
sions exhibited  on  the  stage,  which  have  been  described  in 
a  former  chapter ;  for,  as  it  was  there  explained,  it  is  only 
a  part  of  the  light  that  falls  upon  a  glass  plate  that  is  re- 
flected from  it,  and,  consequently,  any  object  that  is  to  be 
seen  by  reflection  must  be  strongly  illuminated. 

This  is  especially  the  case  when,  as  has  already  been  ex- 
plained, a  double  reflection  is  required  to  produce  the  de- 
sired effect  in  the  best  manner.  You  will  recollect  that, 
by  one  reflection  only,  in  an  inclined  glass,  objects  that  are 
perpendicular  in  reality  are  made  to  appear  horizontal.  To 
remedy  this  difficulty,  and  bring  the  image  into  a  right  po- 
sition, a  second  reflection  is  necessary.  When,  in  order  to 
reflect  this,  two  plates  of  glass  are  used,  as  shown  in  the 
last  chapter,  a  specially  bright  light  is  required  to  supply 
the  necessary  quantity  for  the  double  reflection. 
F 


122  VERY    BKIGHT   LIGHTS. 

You  must  understand,  however,  that,  as  was  explained 
in  describing  such  spectral  illusions,  nothing  of  all  these 
arrangements  and  effects  is  seen  by  the  spectators  in  front, 
except  the  ultimate  image  seen  in  the  upper  glass,  and  ap- 
pearing as  if  it  stood  upon  the  stage.  The  plates  of  glass  j 
the  course  of  the  rays,  from  the  source  of  light  in  the  in- 
strument where  the  man  is  sitting,  through  its  zigzag 
course  to  the  eyes  of  the  spectators ;  the  two  images,  one 
on  the  lower  and  one  on  the  upper  glass,  as  well  as  the 
form  and  position  of  the  glasses  themselves,  are  all  shown 
in  the  engraving  of  this  double  reflection  for  the  purpose 
of  showing  what  the  actual  track  of  the  rays  through  the 
air  is  in  such  a  case.  But  we  never  really  see  rays  passing 
thus  through  the  air  before  us.  The  eye  takes  no  cognizance 
of  any  rays  except  those  which  actually  enter  it,  and  are 
concentrated  by  the  lens  into  an  image  upon  the -retina. 
Thus  the  spectators,  in  the  case  of  an  illusion  like  this, 
would  be  wholly  unconscious  of  all  these  movements  of  the 
light,  and  even  of  the  existence  of  the  glasses,  although  one 
of  them  would  be  full  before  them.  The  light  would  only 
enter  their  eyes  as  it  was  reflected  the  last  time,  which 
would  be  exactly  as  if  it  came  from  a  figure  standing  behind 
the  glass  upon  the  stage,  and  thus  the  illusion  is  created. 

In  other  words,  the  ghost  seen  is  simply  the  reflection 
of  a  real  figure  in  a  mirror.  In  ordinary  cases  we  know 
that  the  reflection  seen  in  a  mirror  is  an  illusion,  for  the 
mirror  is  silvered  on  the  back  so  as  to  allow  no  light  from 
any  thing  really  behind  it  to  pass  through,  and  thus  only 
the  objects  that  are  reflected  in  it  can  be  seen.  We  see  the 
frame  of  it,  moreover,  so  that  we  know  that  the  mirror  is 
there.  But,  in  the  case  of  these  spectres,  the  plates  of 
glass  have  no  frames,  and  the  edges  of  them  are  concealed, 
and  we  see,  moreover,  objects  through  the  glass  as  well  as 
those  reflected  in  it.  In  ordinary  cases,  when  we  see  ob 


EXPERIMENT   AT   THE    WINDOW.  123 

jects  through  a  glass,  we  do  not  see  those  reflected  in  it, 
because  the  light  shining  on  the  objects  beyond  that  are 
seen  through  the  glass  is  usually  sufficient  to  overpower, 
or  nearly  overpower,  the  reflected  light ;  but,  by  throwing 
a  very  strong  light  upon  any  object  that  is  to  be  reflected, 
we  can  remedy  this,  and  enable  ourselves  to  see  the  image 
of  the  illuminated  object  by  reflection  as  plainly  as  we  do 
those  beyond  the  glass  directly,  as  can  be  shown  in  a  very 
simple  and  conclusive  manner  by  the  experiment  ah'eady 
explained  of  holding  a  piece  of  paper,  with  a  lamp  or  can- 
dle shining  directly  upon  it,  near  a  pane  of  glass  in  the 
window  in  the  daytime.  The  paper  thus  illuminated  will 
be  very  distinctly  seen  reflected  in  the  glass.  Indeed, 
white  paper  emits  usually  so  much  light  that  it  can  ordi- 
narily be  seen  faintly  reflected  m  the  glass,  if  it  is  held 
near,  without  any  artificial  illumination  ;  but  the  bright- 
ness of  the  image  will  be  greatly  increased  by  the  bright- 
ness of  the  light  shining  upon  it. 

On  the  same  principle,  if  you  stand  near  a  window,  with 
your  back  toward  it,  and  hold  up  a  pane  of  glass,  or  any 
small  piece  of  glass,  before  your  eyes,  you  will  see  the  ob- 
jects out  of  doors  very  plainly  reflected  in  it,  especially  if 
it  is  a  bright  day.  You  can  also  see  through  the  glass  the 
objects  that  are  in  the  room,  but  the  objects  outside  will 
be  seen  too,  very  distinctly,  and  the  more  distinctly  in  pro- 
portion to  the  brightness  of  the  light  which  shines  upon 
them. 

Thus,  when  for  any  reason  we  wish  to  see  any  object  dis- 
tinctly by  reflected  light  in  a  glass  which  is  not  silvered,  we 
require  a  very  bright  light  to  shine  upon  it,  and  this  is  con- 
sequently one  of  the  purposes  for  which  a  very  bright  light 
is  required. 

On  what  principle  and  by  what  methods  these  very  bright 
lights  are  obtained  will  appear  in  the  next  chapters. 


124  COMBUSTION    OF    MAGNESIUM. 


CHAPTER  XIV. 

COMBUSTION    OF   MAGNESIUM. 

ONE  day,  just  before  the  time  for  dinner,  John  came 
home  from  a  ramble  which  he  had  been  taking  through 
the  streets  in  London.  The  table  was  set  for  dinner,  and 
Lawrence  was  reading  a  newspaper,  having  comfortably 
established  himself  in  a  large  arm-chair  near  a  window. 

When  Lawrence  heard  the  rap  which  John  gave  at  the 
knocker  at  the  door,  he  said, 

"  There  comes  John." 

He  knew  him  by  his  rap. 

It  is  surprising  how  many  different  modes  there  are  in 
use  among  mankind  for  communicating  ideas  and  intelli- 
gence as  substitutes  for  language.  One  very  striking  in- 
stance is  that  of  the  boatswain's  pipe,  on  board  ships  at 
sea,  which  interested  John  very  much  on  his  voyage  across 
the  Atlantic  in  the  steamer.  The  boatswain,  as  perhaps 
the  reader  knows,  is  the  officer  on  board  a  ship  who  has 
charge  of  the  sails  and  rigging;  and  as  the  winds  and 
waves  are  often  so  boisterous  that  no  human  voice  could 
be  heard  at  the  distance  at  which  commands  often  have  to 
be  given,  the  custom  has  grown  up  among  all  European 
nations  of  the  boatswain's  giving  his  directions  to  the  men 
by  means  of  a  peculiar  kind  of  whistle,  called  the  boat- 
swain's pipe,  which  makes  a  very  shrill  and  piercing  sound, 
not  loud,  but  so  penetrating  that  it  can  be  heard  in  the 
stormiest  times  above,  or,  rather,  through  the  sound  of  the 
heaviest  roaring  and  thundering  of  the  winds  and  waves. 

John  had  been  quite  surprised  dui'ing  the  voyage  at  two 


SIGNIFICANCE    OF   KNOCKING.  125 

things  in  respect  to  the  boatswain's  pipe :  first,  at  the  dis- 
tinctness with  which  a  sound  so  slender  and  thin  could  be 
heard  amidst  the  wildest  commotion  of  the  elements,  and 
also  at  the  great  variety  of  commands  that  the  boatswain 
could  give  with  his  pipe,  by  means  of  variations  and  mod- 
ulations in  the  tone  of  it,  which  he  made  by  the  motion  of 
his  hand  over  a  little  hole  in  a  part  of  the  pipe  from  which 
the  air  issued.  The  number  and  variety  of  the  orders  and 
directions  that  he  could  give  by  this  means  constituted 
quite  a  language. 

In  the  same  way,  when  he  landed  in  England,  John  was 
much  interested  and  amused  in  observing  to  how  great  an 
extent  the  knockers  on  the  doors  were  used  as  a  means  of 
communicating  intelligence.  A  single  blow  with  a  knock- 
er is  the  rap  of  "  tradespeople,"  as  they  call  them — that  is, 
porters  bringing  parcels,  or  messengers  from  the  butchers 
or  grocers,  or  persons  having  any  thing  to  sell.  A  double 
rap  is  reserved  exclusively  for  the  postman.  When  that 
sound  is  heard  everybody  in  the  house  knows  that  the  let- 
ters have  come,  and  the  person  that  attends  the  door  must 
go  at  once,  so  as  not  to  keep  so  important  an  official  wait- 
ing. The  two  strokes  generally  come  very  close  together 
— rat-tat — as  quick  almost  as  you  could  possibly  speak 
those  two  syllables ;  but,  however  rapidly  they  are  given, 
so  long  as  there  are  two,  every  body  knows  that  it  is  the 
postman.  Then,  if  it  is  a  gentleman  or  lady,  whether  be- 
longing to  the  house  or  a  visitor,  there  is  quite  a  prolonged 
rapping,  the  strokes  being  usually  quite  rapid  at  first,  and 
more  deliberate  and  emphatic  at  the  end — more  or  less  so 
according  to  the  rank  and  importance  of  the  person  knock- 
ing, thus,  Rat-tat-tat-a-tat-tat-tat,  tat!  TAT! 

Now,  in  beating  such  a  tattoo  as  this  with  the  knocker 
of  the  door,  no  two  persons,  of  course,  do  it  alike,  and  the 
length  and  complicateness  of  the  series  of  strokes  admits 


126  COMBUSTION    OF   MAGNESIUM. 

of  so  much  variety  in  the  knocks  of  different  persons,  with- 
out  the  danger  of  any  of  them  being  confounded  with  the 
ti-adesrnen's  or  postman's  knock,  that  almost  every  indi- 
vidual comes  to  have  his  own  peculiar  rap,  and  thus  the 
knocker  has  a  language,  as  it  were,  of  its  own,  notifying 
those  who  are  in  the  house  of  the  rank  and  position  of  the 
person  at  the  door,  and,  in  case  it  is  any  inmate  of  the  fam- 
ily, making  it  known  at  once  who  it  is. 

And  this  is  how  it  happened  that  Lawrence,  on  hearing 
the  knocker  sounded  at  the  door  while  he  was  reading  his 
newspaper,  said  at  once, "  Here  comes  John." 

All  this  explanation,  however,  of  the  language  of  the 
boatswain's  whistle  at  sea,  and  of  the  knocker  on  the  doors 
of  English  houses,  is  a  digression,  and  it  would  be  some- 
what irrelevant  in  a  scientific  treatise  on  light  were  it  not 
that  a  language  belonging  to  this  same  class,  and  of  substan- 
tially the  same  character  in  respect  to  its  principles,  and  of, 
perhaps,  about  the  same  scope  as  to  copiousness  and  ex- 
tent, has  gradually  grown  up  among  the  light-houses  on 
sea-coasts,  by  means  of  which  some  simple  but  very  im- 
portant information  can  be  communicated  to  ships  at  sea 
through  variations  in  the  light,  as  will  hereafter  be  more 
particularly  explained. 

John  very  soon  came  into  the  room,  and  as  he  entered 
the  door  he  held  up  a  small  object  between  his  thumb  and 
finger. 

"  See,"  said  he. 

Lawrence  looked  up.  John  advanced  toward  him,  hold- 
ing out  what  had  much  the  appearance  of  a  watch-spring 
coiled  up,  except  that  the  color  was  of  a  bluish-gray. 

"What  is  it?"  asked  Lawrence. 

"  Magnesium,"  replied  John.  "  It  is  a  yard  long  when  it 
is  uncoiled.  I  bought  it  for  sixpence." 

"  That's  cheap,"  said  Lawrence. 


FUMES   AND    OTHER   PRODUCTS.  127 

"Yes,"  said  John;  "of  course  I  mean  an  English  six- 
pence. I  saw  it  in  a  window,  and  I  went  in  and  bought 
some  of  it ;  I  am  going  to  burn  it,  and  make  a  bright  light ; 
of  course  I  am  not  going  to  burn  it  all  at  once,  here ;  I  am 
only  going  to  burn  a  small  piece — half  an  inch  long,  per- 
haps, just  for  an  experiment,  and  the  rest  I'm  going  to  take 
to  America." 

Lawrence  approved  of  this  arrangement,  and  it  was 
agreed  that  they  would  try  the  experiment  that  evening 
after  dinner. 

There  was  some  question  about  the  fumes  which  might 
arise,  but  Lawrence  said  he  thought  that  there  would  be 
no  fumes,  as  the  product  of  the  combustion  of  magnesium 
was  simply  magnesia,  which  was  a  harmless  white  powder; 
in  other  words,  a  finely  comminuted  solid.  Fumes  arose 
from  combustion,  he  said,  only  when  the  products,  or  some 
of  them,  were  gaseous,  so  that  they  might  rise  and  float  in 
the  air. 

It  is  true  that  sometimes,  when  the  products  of  the  com- 
bustion, or  the  substances  set  free  by  it  are  solid,  they  are 
developed  in  the  form  of  a  powder  so  fine  as  to  be  borne 
upward  by  the  current  of  hot  air,  so  as  to  produce  the  ap- 
pearance of  fumes,  and  sometimes  they  mingle  with  true 
fumes  actually  produced.  This  happens  very  strikingly  in 
the  case  of  the  combustion  of  wood  or  coal,  in  which  very 
fine  particles  of  carbon,  detached  from  the  substance  of  the 
wood  or  coal,  are  carried  up  among  the  fumes  of  carbonic 
acid  gas  and  the  vapor  of  water,  which  are  really  the  pro- 
ducts of  the  combustion. 

Now  combustion,  as  probably  the  readers  of  this  book 
remember,  is  only  the  rapid  combination  of  a  substance 
with  the  element  called  oxygen,  which  exists  abundantly 
in  the  air,  and  has  such  an  eager  affinity  for  many  other 
substances,  especially  when  they  are  heated  up  to  a  certain 


128  COMBUSTION    OF   MAGNESIUM. 

Doint,  as  to  combine  with  them  with  great  rapidity  and 
violence.  In  doing  this  they  develop  or  expend  so  much 
force  as  to  produce  a  great  quantity  of  light  and  heat, 
which  are  considered  as  only  two  of  the  many  forms  of 
force.  To  commence  this  process  of  rapid  combination 
with  oxygen,  a  portion  of  the  substance  must  first  be  heat- 
ed to  the  requisite  point ;  but,  when  it  is  once  commenced., 
it  goes  on,  the  heat  developed  by  the  combustion  raising 
the  successive  portions  to  the  right  temperature  for  con- 
tinuing the  process.  This  heating  a  portion  of  the  com- 
bustible in  order  to  commence  the  process  is  what  we  call 
kindling  the  fire.  All  this  has  already  been  explained,  and 
must  not  be  forgotten. 

Now,  when  substances  are  burned — that  is,  are  delivered 
over  to  this  eager  and  fierce  seizure  of  their  particles  by 
oxygen,  the  compounds  that  are  produced  are  called  the 
products  of  combustion,  and  these  products,  of  course,  vary 
very  much  according  to  the  nature  of  the  substances  com- 
bined. Sometimes  they  are  gases  which  rise  into  the  air. 
Sometimes  they  are  powdered  solids.  In  the  case  of  mag- 
nesium, the  product  is  the  well-known  white  powder  mag- 
nesia, which  is,  in  chemical  language,  the  oxide  of  magne- 
sium, or,  as  it  now  is  sometimes  proposed  to  call  it,  magne- 
sium oxide. 

John  knew  all  this,  so  that  when  Lawrence  told  him 
there  Avould  be  no  danger  from  fumes  in  burning  his  mag- 
nesium, he  was  ready  to  assent  to  it  at  once. 

"But,  then,"  said  Lawrence, "there  is  sometimes  a  possi- 
bility that  some  fused  portion  of  the  substance  to  be  burned 
may  fall  down,  and  do  harm  in  that  way.  This  happens 
when  the  heat  produced  melts  the  substance  faster  than 
there  is  oxygen  at  hand  to  combine  with  it.  I  do  not  know 
how  this  may  be  with  magnesium,  and  so,  in  order  to  make 
our  experiment  perfectly  safe,  we  will  ask  the  landlady  to 


ASHES.  129 

lend  us  an  old  kitchen  plate,  and  burn  our  magnesium  over 
that." 

This  was  accordingly  done.  The  landlady,  when  the 
table  was  cleared,  brought  in  a  plate.  John  broke  off 
about  an  inch  in  length  from  the  end  of  his  little  ribbon 
of  magnesium,  and  for  a  handle  he  used  a  match,  first 
breaking  off  the  phosphoric  end,  and  then  making  a  little 
cleft  with  his  pocket  knife  in  the  wood,  by  which  means 
he  formed  a  kind  of  extemporized  forceps  to  hold  the  mag- 
nesium. When  all  was  ready,  Lawrence  lighted  another 
match  and  set  the  end  of  the  magnesium  on  fire,  while 
John  held  it  over  the  plate.  It  kindled  with  some  difficul- 
ty, as  if  the  end  of  the  metal  required  to  be  raised  to  a 
great  heat  before  the  process  of  combining  with  the  oxy- 
gen of  the  air  could  begin ;  but,  when  it  was  once  begun,  it 
went  on  with  a  very  intense  action,  producing  a  light  so 
vivid  and  dazzling  that  it  was  almost  impossible  to  look 
at  it. 

The  piece  of  magnesium  which  was  burnt  was  very  short, 
and  it  was,  moreover,  very  narrow  and  exceedingly  thin, 
so  that  it  was  soon  expended.  John  uttered  some  excla- 
mations of  delight  while  the  burning  was  going  on,  and 
when  it  went  out  he  looked  attentively  at  what  Avas  left. 
It  was  a  white  substance  of  exactly  the  same  form  with 
the  little  ribbon  of  magnesium,  but,  on  touching  it,  it  fell 
to  an  impalpable  powder. 

"  What  white  ashes  !"  said  John. 

"  No,"  replied  Lawrence,  "  that  is  not  properly  ashes  at 
all.  The  ashes  left  in  burning  wood  are  not  produced  by 
tiie  combustion,  but  only  left  by  it.  The  substances  which 
are  produced  by  the  combustion  in  the  case  of  wood  go  off 
into  the  air  as  gases;  the  ash  is  only  the  incombustible 
substance  that  is  left  behind.  But  the  white  powder  in 
this  case  is  formed  by  the  combustion — that  is,  it  is  com- 
F2 


130  COMBUSTION    OF   MAGNESIUM. 

posed  of  the  magnesium  itself,  combined  with  the  oxy- 
gen." 

"  Yes,"  said  John, "  I  know.     It  is  magnesia." 

"It  is  well  enough  to  call  it  the  ashes  in  common  par- 
lance," continued  Lawrence, "  on  account  of  its  resemblance 
to  the  ashes  of  wood  or  paper  in  its  apparent  origin  and  in 
its  form,  if  we  only  know  that  it  is  formed,  chemically,  in 
quite  a  different  way." 

To  have  been  perfectly  precise  in  his  statement,  Law- 
rence might  have  added  that  the  ash  left  in  the  burning  of 
wood  is  mostly  composed  of  compounds  of  certain  metals 
with  oxygen,  formed  by  some  previous  process  analogous 
to  combustion,  and  left  in  the  ground,  whence  they  were 
taken  up  by  the  rootlets  of  the  plants,  and  built,  so  to 
speak,  into  the  wood.  But  the  combustion,  if  it  really  was 
a  process  of  combustion,  by  which  they  were  originally 
produced,  was  not  the  burning  of  the  wood,  but  took  place 
long  before.  In  the  combustion  of  the  wood  they  were 
simply  passed  over  and  left,  whereas,  in  case  of  magnesium, 
the  magnesia  which  results  is  produced  at  the  time,  and  by 
the  very  process  of  the  burning. 

"It  did  not  drop  upon  the  plate  after  all,"  said  John, 
looking  at  the  plate,  which  remained  perfectly  clean  after 
the  experiment. 

"No,"  replied  Lawrence;  "I  was  almost  sure  that  it 
would  not.  I  wras  very  confident  that  the  burning  would 
keep  well  in  advance  of  any  tendency  to  melting ;  but,  in 
trying  philosophical  experiments  in  a  parlor,  it  is  always 
best  to  take  measures  for  guarding  against  even  the  most 
improbable  contingencies." 


BED   HEAT  AND   WHITE   HEAT.  131 


CHAPTER  XV. 

THE    MAGNESIUM   LAMP. 

THE  brightness  of  the  light  produced  in  any  sense  by 
combustion  seems  to  depend  upon  two  things — first,  the 
intensity  of  the  heat  developed  by  the  combustion ;  and, 
secondly,  upon  the  presence  of  solid  particles  to  be  raised 
to  what  is  called  a  white  heat  by  this  intensity.  Gaseous 
substances,  though  the  heat  may  be  very  great,  emit  usual- 
ly a  comparatively  faint  light,  as  is  observed  in  the  case  of 
the  flame  of  hydrogen  or  of  alcohol,  which  substances  in 
combustion,  though  the  heat  produced  is  very  great,  give 
rise  chiefly  to  incandescent  gases.  But  in  the  case  of  mag- 
nesium there  is  not  only  a  Very  intense  heat,  but  this  heat 
takes  effect  upon  the  solid  particles  of  magnesia  as  fast  as 
they  are  produced,  and  causes  them  to  emit  a  light  of  the 
greatest  possible  brilliancy. 

When  any  solid  is  heated  in  a  furnace,  we  observe  that 
it  first  begins  to  emit  a  reddish-colored  light,  or,  as  we  say, 
it  becomes  red  hot.  When  the  heat  is  raised  to  a  much 
higher  degree,  the  light  that  radiates  from  it  becomes 
brighter  and  whiter,  and  we  say  it  is  white  hot.  This 
would  seem  to  be  the  secret  of  the  very  intense  light 
given  out  by  the  combustion  of  magnesium.  The  com- 
bustion produces  an  extremely  high  degree  of  heat,  and 
this  takes  effect  on  the  solid  particles  of  magnesia  as  fast 
as  they  are  produced,  raises  them  to  the  most  intense  in- 
candescence, and  causes  them  to  emit  the  very  brilliant 
and  dazzling  radiation  which  we  see. 

This  white  heat,  moreover,  is  not  only  different  in  degree 


132 


THE   MAGNESIUM   LAMP. 


from  the  reel  heat,  but  it  seems  to  be,  in  some  way,  of  a  dif- 
ferent kind — at  least  it  is  found  capable  of  producing  dif- 
ferent effects,  and  it  is  in  consequence  of  these  peculiar  ef- 
fects that  the  magnesium  can  be  made  very  useful  for  cer- 
tain philosophical  purposes.  One  would  suppose  that  it 
would  be  very  difficult  to  devise  a  lamp  for  burning  a  solid 
metal  in  the  form  of  a  ribbon  of  wire,  or,  indeed,  in  any 
other  form,  but  the  difficulty  has  been  surmounted  in  va- 
rious ways.  One  of  the  modes  by  which  this  has  been  ac- 
complished is  shown  in  the  engraving.  The  instrument  is 
called  the  magnesium  lamp. 


The  metal  is  used  in  the  form  of  a  wire,  which  is  wound 
upon  the  wheel  A,  which  wheel  thus  takes  the  place  of  the 
reservoir  containing  the  oil  in  a  common  lamp.  The  wire 
is  drawn  off  from  the  wheel  slowly  by  clock-work  contained 
in  the  box  B;  within  the  'box  it  passes  between  two  wheels 


USES    OF   THE    MAGNESIUM   LIGHT.  133 

made  of  gutta-percha,  or  fitted  with  gutta-percha  surfaces, 
which  substance  holds  it  with  a  sufficiently  firm  grasp  to 
draw  it  forward  between  them  as  the  wheels  revolve.  G 
is  the  key  by  which  the  clock-work  is  wound  up  when  it 
runs  down,  and  at  T  is  the  tongue  of  a  little  catch  by 
means  of  which  the  clock-work  may  be  set  going  or 
stopped  at  pleasure.  The  wire  of  magnesium  is  burned 
at  the  end  C,  which  protrudes  in  front  of  the  concave  mir- 
ror, being  pushed  forward  by  the  clock-work  as  fast  as  it 
burns,  while  the  magnesia  that  results  from  the  combus- 
tion falls  down  into  the  pan  E  below.  F  is  a  thumb-screw 
connected  with  rack -work,  by  which  the  mirror  can  be 
moved  backward  or  forward  as  required.  The  whole  can 
be  taken  up  by  the  handle,  which  serves,  when  the  lamp  is 
stationary,  as  one  of  the  legs. 

The  magnesium  light  is  used  chiefly  as  a  substitute  for 
the  light  of  the  sun  in  photography,  especially  in  cases 
where  the  light  of  the  sun  is  not  at  command,  as,  for  in- 
stance, in  caverns,  and  mines,  and  other  dark  places.  Its 
intensity,  and  certain  chemical  properties  which  result 
from,  or,  at  least,  accompany  this  intensity,  fit  it  to  answer 
these  purposes  extremely  well. 

It  has  been  used  in  this  way  very  successfully  in  photo- 
graphing interior  views  of  the  great  pyramid  in  Egypt,  and 
in  many  other  similar  cases,  where  none  but  artificial  light 
could  possibly  be  obtained.  It  is  also  sometimes  used  for 
engineering  and  military  purposes,  such  as  for  illuminating 
works  of  construction  when  it  becomes  necessary  to  carry 
them  on  at  night,  and  also  for  showing  the  position  and 
movements  of  the  enemy  in  case  of  nocturnal  operations  in 
war.  When,  for  example,  the  garrison  of  a  besieged  city 
wish  to  make  a  sortie  at  night,  if  they  can  send  off  in  ad- 
vance, or  at  a  little  distance  from  them  on  one  side,  an  in- 
tensely brilliant  light,  their  enterprise  is  greatly  aided,  and 


134  THE    MAGNESIUM   LAMP. 

that  in  two  ways.  The  light,  while  they  themselves  re- 
main behind  it  in  the  shade,  shows  them  the  enemy  and 
the  defenses,  if  there  are  any,  which  they  are  to  attack, 
very  clearly,  and  at  the  same  time  dazzles  the  eyes  of  the 
enemy,  bewilders  their  vision,  and  confuses  their  aim. 

In  order  fully  to  understand  what  is  to  follow,  the  reader 
must  not  lose  sight  of  the  principle  on  which  the  magne- 
sium light  is  produced — namely,  by  the  intense  avidity 
with  which  the  oxygen  of  the  air  seeks  to  enter  into  re- 
combination with  the  magnesium,  from  which  it  was  sep- 
arated by  the  use  of  great  force  when  the  metal  was  pre- 
pared, and  the  consequent  heat,  which  raises  the  solid  par- 
ticles of  magnesia  to  a  dazzling  incandescence  as  fast  as 
they  are  formed.  Magnesium  is  never  found  in  its  metal- 
lic form  in  nature.  It  is  always  found  already  in  combina- 
tion with  oxygen,  either  in  magnesia,  which  is  the  simple 
oxyde,  or  in  some  other  form  or  combination  in  which  it  is 
already  oxydated;  and  the  oxygen  with  which  it  is  com- 
bined clings  to  it  with  such  tenacity  that  it  requires  a  very 
great  chemical  force  to  separate  it,  so  as  to  produce  the 
metal  in  a  pure  and  isolated  state. 

I  mean  by  a  great  chemical  force  a  force  which,  though 
really  very  great,  is  exercised  within  such  extremely  small 
limits  in  respect  to  distance  as  to  be  entirely  unapprecia- 
ble  by  the  senses.  We  have  an  example  of  a  force  in  some 
respects  analogous  to  this  in  the  freezing  of  water,  by 
which  the  particles  are  forced  apart  only  to  an  inconceiv- 
ably minute  distance  from  each  other,  but  yet  with  so  much 
force  as  to  lift  and  displace  the  heaviest  walls  if  they  rest 
upon  ground  that  the  frost  can  reach,  or  to  break  asunder 
the  strongest  vessels  when  the  freezing  water  is  confined 
in  them  ;  and  so,  also,  with  the  force  with  which  the  juices 
are  drawn  up  in  the  vessels  of  plants  and  trees  in  the  pro- 
cess of  vegetation.  This  force,  though  inappreciable  to  our 


DIFFICULTY    OF    OBTAINING   MAGNESIUM.  135 

senses,  is  sufficient  to  move  the  heaviest  stones,  to  lift  and 
tear  up  pavements,  and  to  push  up  and  sustain  the  materi- 
als of  which  the  branches  and  leaves  of  the  tree  are  com- 
posed, hundreds  of  feet  into  the  air. 

It  is  by  a  force  somewhat  analogous  to  these  in  respect  to 
the  minuteness  of  the  limits  through  which  it  operates,  and 
the  vastness  of  the  power  which  it  exerts  within  those  lim- 
its, that  the  particles  of  the  metallic  magnesium  are  held 
in  combination  with  those  of  oxygen  in  all  the  substances 
in  which  it  is  found  in  a  state  of  nature.  And  so  firmly  is 
it  held  by  this  force,  that,  though  innumerable  experiments 
were  made  with  the  substances  in  which  it  was  combined, 
it  was  a  very  long  time  before  the  existence  of  the  hidden 
metal  in  these  substances  was  discovered.  The  discovery 
was  at  length  made  in  1827.  Small  portions  were  separa- 
ted, and  the  metal,  as  a  metal,  brought  to  view ;  but  it  was 
not  until  quite  recently  that  methods  were  devised  by 
which  any  great  quantities  could  be  produced. 

Of  course,  in  these  attempts,  the  substance  of  the  magne- 
sium could  be  brought  into  its  metallic  form  only  by  sep- 
arating the  oxygen  from  it,  and  this  could  be  done  only  by 
applying  a  greater  force  to  the  oxygen  than  that  by  which 
it  was  united  with  the  magnesium.  This  force  was,  as  has 
already  been  said,  very  great.  Indeed,  the  eagerness  with 
which  it  returns  to  the  combination,  and  which  is  the  cause 
of  the  great  development  of  heat  and  light,  is  the  measure 
of  this  force.  Thus  the  chemist,  in  separating  the  magne- 
sium from  its  oxygen  in  its  natural  combinations,  forces 
the  substances  apart  for  the  sake  of  witnessing  the  effects 
produced  by  the  violence  with  which  they  come  together 
again.  The  operation  is  very  analogous  to  that  of  lifting 
a  stone  high  into  the  air  in  order  to  observe  the  force  of 
the  concussion  with  which  it  strikes  the  ground  in  falling. 


136  INCANDESCENCE. 


CHAPTER  XVI. 

INCANDESCENCE. 

THERE  are  various  methods  by  which  an  intense  white 
light  is  produced  by  artificial  means,  but  most  of  them,  if 
not  all,  depend  on  the  same  principle  as  that  already  ex- 
plained in  the  case  of  the  magnesium  light — that  is,  in  rais- 
ing particles  of  a  solid  substance  to  incandescence.  The 
two  essentials  are,  first,  some  method  of  producing  an  in- 
tense heat ;  and,  secondly,  the  presence  of  some  solid  sub- 
stance to  receive  the  heat  and  to  emit  the  light  developed 
by  it ;  for  light,  for  some  mysterious  reason,  is  emitted 
much  more  powerfully  from  a  solid  substance,  however 
minutely  subdivided,  than  from  a  gas. 

The  same  general  principle,  indeed,  is  seen  to  operate  in 
the  case  of  light  derived  from  the  lower,  as  well  as  in  that 
from  the  higher  temperatures  produced  by  combustion. 
This  is  shown  quite  clearly  in  the  flame  of  a  common  lamp 
or  candle. 

The  materials  used  for  burning  in  lamps  and  candles,  as 
the  reader  will  recollect,  usually  belong  to  a  class  of  sub- 
stances called  hydrocarbons.  They  are  so  called  because 
they  are  chiefly  composed  of  hydrogen  and  carbon.  Their 
burning  is,  of  course,  the  combination  of  these  substances 
with  the  oxygen  of  the  surrounding  atmosphere. 

Now  hydrogen,  in  combining  with  oxygen,  produces,  un- 
der favorable  circumstances,  a  very  intense  heat,  and  forms 
by  the  combination  the  vapor  of  water.  This  vapor  rises 
from  the  flame  and  is  diffused  through  the  atmosphere. 
"We  do  not  see  it  as  it  arises,  but  we  can  show  it  very  plain- 


WATER   FROM   FIRE.  137 

ly  by  holding  a  cold  iron  over  the  flame,  at  a  little  distance 
above  it,  when  we  shall  find  it  will  be  almost  immediately 
covered  by  a  dew  formed  by  the  condensation  of  the  vapor 
into  a  film  of  exceedingly  minute  drops  of  liquid  water. 

And  so,  when  you  light  a  lamp  in  cold  weather,  the  glass 
chimney,  if  put  on  cold,  becomes  for  a  moment  bedimmed 
with  a  dew  produced  by  the  condensation  of  the  aqueous 
vapor  formed  by  the  combustion  of  the  hydrogen.  As 
soon  as  the  glass  becomes  warm  the  vapor  is  no  longer 
condensed,  though  it  continues  to  be  formed  as  before. 

This  phenomenon  may  be  shown 
in  a  still  more  perfect  manner  by 
burning  a  candle  for  a  few  minutes 
under  a  cold  bell  glass,  and  observ- 
ing the  deposition  of  the  water  on 
the  interior  of  the  glass,  which  will 
sometimes  be  so  abundant  as  to 
cause  drops  to  trickle  down  the 


ATKU   FROM   FIKE. 


This  experiment  of  condensing  water  from  the  products 
of  flame,  which  any  one  can  easily  perform,  will  succeed 
better  if  the  iron,  or  other  condensing  substance,  has  some 
thickness,  so  as  not  to  become  warmed  itself  too  soon,  and 
so  cease  to  condense  the  vapor;  and  if  it  has  also  a  polished 
surface,  as  such  a  surface,  by  its  brightness  being  dimmed, 
will  show  the  presence  of  very  small  quantities  of  vapor. 

Sometimes  children,  when  they  are  writing  a  letter,  and 
are  in  haste  for  the  writing  to  dry,  hold  it  at  a  distance 
over  the  flame  of  a  lamp,  not  knowing  that  the  hydrogen, 
which  forms  a  large  part  of  the  oil,  produces  water  by  its 
combination  with  oxygen  in  the  burning,  and  that  this 
water,  in  the  form  of  vapor,  rises  directly  to  the  place 
where  they  are  holding  their  writing  to  dry  !  In  other 
words,  they  hold  their  paper  in  a  very  damp,  though  in  a 


138  INCANDESCENCE. 

very  hot  place,  as  is  shown  at  once  by  holding  there  a  cold 
chisel,  or  hatchet,  or  large  carving-knife,  or  any  other  piece 
of  polished  and  cold  iron. 

The  other  substance  contained  in  the  hydrocarbon  burned 
in  the  lamp  or  candle  is  carbon.  This,  so  far  as  it  is  really 
burned — that  is,  so  far  as  it  finds  oxygen  to  unite  with  it, 
forms  a  suffocating  gas,  called  carbonic  acid  gas,  which  it 
is  very  injurious  to  breathe.  This  gas  rises  with  the  vapor 
of  water  into  the  air,  and  is  diffused  in  the  upper  part  of 
the  room  till  it  gets  cold,  when  it  descends  and  gradually 
escapes  through  open  doors,  or  windows,  or  up  the  chim- 
ney. If  there  is  no  way  of  escape  for  it  open,  or  if  there 
are  many  lamps,  or  candles,  or  gas  jets  burning  in  the  room, 
the  air  becomes  gradually  so  charged  with  it  as  to  be  un- 
comfortable and  unhealthy  to  breathe. 

But  all  the  carbon  does  not  at  once  find  oxygen  enough 
at  hand  to  combine  with  it.  A  portion  of  it  remains  for  a 
moment  in  the  flame,  where  it  serves  the  purpose  of  fur- 
nishing a  supply  of  solid  particles  to  emit  light.  They  can 
not  all  burn  at  once,  because  there  is  not  oxygen  enough 
for  all ;  so  that,  while  some  are  burning,  and  evolving  great 
heat  in  so  doing,  the  others,  while  waiting  their  turn  to  be 
supplied  with  oxygen,  as  if  not  willing  to  be  idle  and  use- 
less even  for  a  moment,  employ  themselves  in  producing 
and  emitting  light,  which  the  heat  that  is  supplied  to  them 
empowers  them  to  do.  At  length,  however,  when  they 
reach  the  upper  and  outer  margin  of  the  flame,  they  too 
obtain  their  supply  of  oxygen  from  the  air,  and,  combining 
with  it,  give  out  more  heat,  and  also  form  more  carbonic 
acid  gas,  which  arises  with  the  rest  into  the  air. 

Thus,  on  their  way  through  the  flame,  after  being  liber- 
ated from  their  previous  combination  with  hydrogen  in  the 
hydrocarbon,  and  before  their  turn  comes  to  be  supplied 
with  oxygen  to  enable  them  to  form  a  new  combination, 


CARBOX    IX    THE    FLAME.  139 

they  serve  as  solid  particles,  to  emit  light  by  their  incan- 
descence. 

It  is  from  these  solid  particles — individually  solid,  though 
inconceivably  minute — that  the  chief  portion  of  the  light 
of  such  a  flame  comes.  The  combustion  of  the  hydrogen 
alone,  or  of  any  other  gaseous  substance,  though  it  would 
produce  great  heat,  would  afford  very  little  light.  For 
some  mysterious  reason,  it  is  necessary  that  there  should 
be  solid  particles  present  to  transform,  as  it  were,  a  portion 
of  the  heat  into  light,  and  emit  it  in  that  form. 

These  solid  particles  of  carbon  in  the  flame  are  not  di- 
rectly visible,  but,  as  in  the  case  of  vapor  of  water,  we  can 
easily,  by  the  use  of  proper  means,  bring  them  into  view. 
If,  instead  of  holding  the  cold  iron  in  the  air  above  the 
flame,  we  hold  it,  or  any  other  solid  substance,  actually  in 
the  flame,  the  black  particles  are  suddenly  cooled  by  it,  and 
deposited  upon  its  surface  as  soot  is  upon  the  back  of  the 
chimney.  This  black  substance,  on  account  of  its  being 
produced  in  this  way,  is  called  lamp-black. 

The  process  Avhich  thus  takes  place  in  the  burning  of 
a  candle  is  quite  a  complicated,  and  a  very  curious  one, 
and  if,  in  watching  it,  our  powers  of  vision  were  sufficiently 
acute  to  enable  us  to  distinguish  the  several  steps,  we 
should  be  greatly  interested  in  observing  it.  In  the  first 
place,  we  should  distinguish  in  the  oil,  slowly  coming  up 
the  wick,  particles  of  carbon  and  hydrogen  conjoined.  We 
must  not,  however,  conceive  of  the  particles  of  carbon  as 
black  ;  they  are  black  when  separated  from  their  combina- 
tions and  existing  in  a  certain  form  by  themselves,  but 
they  may  be  of  any  other  color.  Color,  as  will  be  ex- 
plained more  fully  in  a  future  chapter,  depends  altogether 
upon  the  manner  in  which  any  substance  absorbs  or  re- 
fiects  the  light,  and  this  does  not  depend  upon  its  intrinsic 
character  at  all,  but  apparently  upon  the  mechanical  ar- 


140  INCANDESCENCE. 

rangement  of  its  particles.  Thus  sugar,  which  is  white  in 
the  lump,  when  dissolved  in  water  and  diffused  through  it, 
loses  its  whiteness  entirely  and  has  no  color  at  all. 

The  particles  of  carbon  which,  combined  with  the  hydro- 
gen, form  the  oil,  have  only  the  color  of  the  oil  while  in 
this  combination.  When  they  come  up  to  the  flame,  the 
action  of  the  heat,  in  some  mysterious  way  not  at  all  un- 
derstood, has  the  effect  of  developing  in  them  a  strong  ten- 
dency to  separate  from  each  other,  and  to  enter  severally 
into  combination  with  the  oxygen  of  the  air  which  is  near. 
In  combining  with  the  oxygen,  we  should  see  them  seize 
it  with  great  avidity  and  violence,  and  the  force  which 
they  thus  expend  we  should  see  taking  the  form  of  heat, 
which  would  act  upon  the  next  portion  of  oil  which  came 
up,  and  produce  the  same  effect  upon  the  carbon  and  hy- 
drogen in  that ;  and  thus  the  process  would  go  on. 

The  hydrogen  which  was  thus  separated  from  the  oil, 
we  should  see,  would  seize  upon  the  oxygen  with  the  great- 
est avidity,  and  procure  the  largest  share,  or,  at  least,  the 
earliest.  The  carbon  particles  would  have  to  wait,  it  would 
seem,  for  their  supply  until  the  hydrogen  was  satisfied. 
The  consequence  of  this  is,  as  we  should  see,  that  while  the 
hydrogen  combines  at  once  with  the  portion  which  it  re- 
quires, thus  becoming  transformed  into  the  vapor  oftoater, 
the  carbon  particles,  or,  at  least,  a  very  large  portion  of 
them,  pass  up  through  the  flame  intensely  heated,  and,  by 
the  superior  power  of  a  solid  to  radiate  light,  become  the 
source  of  nearly  all  the  light  which  the  flame  affords. 

All  this  we  should  see  if  we  had  senses  acute  enough  to 
perceive  what  really  takes  place  in  the  burning  of  a  lamp 
or  candle. 

The  particles  of  carbon  which  pass  up  thus  through  the 
flame,  though  while  so  hot  they  emit  the  yellow  color  of 
the  flame,  in  other  words,  are  themselves  of  a  yellow  color, 


MAKIXG   LAMP-BLACK.  141 

become  intensely  black  if  they  are  interrupted  on  the  way, 
and  suddenly  cooled  before  they  find  oxygen  to  combine 
themselves  with.  They  are,  moreover,  so  inconceivably 
minute,  that  when  assembled  together  they  form  an  impal- 
pable powder,  far  softer  and  finer  in  the  minuteness  of  the 
division  than  it  would  be  possible  to  make  masses  of  car- 
bon by  any  artificial  process  of  pulverization.  This  mode, 
accordingly,  of  procuring  a  black  powder  for  paint,  and  for 
painter's  work,  is  practically  employed  to  a  great  extent. 
The  engraving  shows  how  lamp-black  is  manufactured  on 
a  large  scale. 

The  fire  is  made  in  the  little  grate  at  «/  it  is  made  of 
pitch,  or  tar,  or  some  other  hydro- 
carbon containing  a  large  propor- 
tion of  carbon.  The  substance  is 
heated  by  a  fire  below  it,  and  then 
is  set  on  fire  above,  and  is  furnished 
with  a  limited  supply  of  oxygen 
through  small  holes  made  for  the 
purpose.  In  consequence  of  this  lim- 
ited supply  of  oxygen,  the  combus- 
tion is  imperfect,  and  a  large  por- 


form  of  a  dense  black  smoke  into  the  chamber  b  c,  where  it 
attaches  itself  to  the  walls,  and  also  to  the  sides  of  the 
cone  <?,  which  is  placed  there  to  receive  it,  and  can  be 
raised  or  lowered  at  pleasure  by  the  cord  and  pulley. 
When  a  sufficient  quantity  of  the  deposit  has  accumulated, 
it  is  removed  from  the  walls  and  the  cone,  and  packed  in 
papers  for  transportation  and  sale. 

This  being  the  philosophy  of  the  light  produced  by  a 
flame  of  a  lamp  —  that  is,  by  the  incandescence  of  solid 
parts  heated  by  the  combustion  and  floating  up  through 
it  —  it  is  plain  that  the  way  to  increase  the  light  is  to  in- 


142  INCANDESCENCE. 

crease  the  rapidity  of  the  combustion,  so  as  to  increase  the 
heat  and  raise  the  solid  particles  to  a  higher  degree  of  in- 
candescence. And  to  increase  the  combustion,  the  proper 
means  is  to  increase  the  supply  of  oxygen.  The  most  ob- 
vious way  of  doing  this  is  to  facilitate  the  access  of  air, 
since  air  aifords  the  most  abundant  natural  supply  of  oxy- 
gen that  we  have  at  command. 

It  was  this  idea  which  Argand  carried  into  practical  ef- 
fect in  his  celebrated  burner,  by  which  he  supplied  a  cur- 
rent of  air  on  the  inside  as  well  as  the  outside  of  the  flame, 
and  also  at  the  same  time  increased  the  rapidity  of  the  sup- 
ply by  creating  a  draft  by  means  of  a  transparent  chimney, 
as  has  already  been  fully  explained. 

The  same  effect  in  principle  is  produced  in  a  common 
form  of  burner  for  gas,  called  the  "  bat's  wing."  This  con- 


TJIE  BAT'S  wi 


sists  in  so  forming  the  opening  for  the  issue  of  the  gas  as 
to  throw  the  flame  into  a  broad,  flat  form,  so  as  to  give  the 
air  access  to  a  very  enlarged  surface  of  it. 

This  method,  like  that  of  Argand,  as  well  as  a  great 


THE    BUDE    LIGHT.  143 

many  other  contrivances  which  act  on  the  same  principle, 
consists  obviously  in  increasing  the  supply  of  oxygen  by 
facilitating  the  access  of  common  air  to  the  combustible 
substances  in  the  flame.  But  common  air  contains  but  a 
small  portion  of  oxygen — one  fifth  only  of  its  volume ;  the 
remainder  consists  of  other  gases,  which  not  only  render 
no  aid  to  the  combustion,  but  positively  impede  it  by  oc- 
cupying the  space  and  keeping  back  the  oxygen  from  ap- 
proach. The  remedy  for  this  difficulty  would  obviously 
be  supplying  the  flame  with  pure  oxygen  instead  of  a  mix- 
ture containing  four  fifths  of  useless  matter.  This  is  done 
in  an  arrangement  which  has  been  quite  celebrated,  and 
which  is  known  as  the  Bude  Light.  This  light  will,  how^ 
ever,  be  more  particularly  referred  to  in  the  next  chapter. 


144  FOLKESTONE. 


CHAPTER  XVH. 

FOLKESTONE. 

WHEN  the  time  arrived  for  crossing  the  Channel  into 
France,  Lawrence  said  to  John,  on  the  morning  of  the  day 
before,  that  he  had  a  bargain  to  propose  to  him. 

"  Good  !"  said  John ;  "  I  agree  to  it  beforehand." 

"  You  know,"  continued  Lawrence,  "  that  there  are  four 
or  five  different  routes  of  travel  from  London  to  Paris, 
crossing  the  Channel  at  different  points.  Now  the  bargain 
which  I  have  to  propose  is,  that  you  shall  choose  the  route 
for  us  to  take,  without  my  having  any  thing  to  say  about 
it,  on  two  conditions." 

"Very  well,"  said  John ;  "  let  us  hear  the  conditions." 

"  The  first  is,"  said  Lawrence, "  that  you  are  not  to  de< 
cide  blindly.  You  are  to  'study  up'  the  subject,  as  they 
say  in  the  guide-books,  and  find  out  what  are  the  relative 
advantages  and  disadvantages  of  the  different  routes." 

"  I  agree  to  that,"  said  John. 

"And  the  second  is,"  continued  Lawrence,  "that  you  are 
to  take  the  whole  charge  of  both  of  us  on  the  passage.     I 
am  to  have  nothing  to  do  but  to  be  quiet  and  do  as  you 
say." 
.     "And  how  about  the  money?"  asked  John. 

"Of  course,"  said  Lawrence,  "I  am  to  put  you  in  funds 
before  you  set  out." 

John  was  much  pleased  with  this  proposition,  and  was 
ready  to  agree  to  it  at  once.  After  a  very  careful  and 
thorough  research  in  the  guide-books,  he  decided  upon  the 
route  through  Folkestone  and  Boulogne. 


CROSSING   THE    CHANNEL.  145 

This  is  one  of  the  two  routes  which  take  you  across  the 
Channel  in  the  narrowest  part,  thus  giving  a  smaller  por- 
tion of  the  journey  to  be  made  by  steamer  on  the  sea 
than  the  other  routes  which  cross  the  Channel  lower  down, 
where  it  is  wider.  The  other  passage  leading  across  the 
narrow  part  of  the  Channel  is  from  Dover  to  Calais.  This 
last  was  far  the  most  usual  route  in  ancient  times,  when 
steam-boats  were  unknown,  and  the  passage  was,  accord- 
ingly, always  made  by  sailing  vessels,  which  did  not  pre- 
tend to  leave  port  at  fixed  hours,  but  only  when  wind  and 
tide  favored. 

Now,  of  these  two  routes,  the  passage  by  Dover  and 
Calais  is  the  most  romantic  in  two  respects — first,  on  ac- 
count of  the  quaint  and  antique  character  of  the  two 
towns,  and  the  many  very  interesting  historical  associa- 
tions connected  with  them;  and,  secondly,  from  the  fact 
that,  as  the  boat  by  that  route  leaves  every  day  at  a  fixed 
hour,  whatever  is  the  state  of  the  tide,  and  as  the  water  in 
the  harbor  there,  as  in  most  of  the  other  harbors  on  the 
shores  of  the  Channel,  is  never  very  deep,  and  is  nearly  all 
out  at  low  tide,  it  happens  that,  when  the  tide  is  low  at  the 
appointed  hour  of  sailing,  the  steamer  necessarily  goes  out 
of  the  harbor  some  time  before,  and  remains  outside,  in  deep 
water,  until  the  hour  arrives,  and  then  the  passengers  go  out 
in  a  small  boat  to  go  on  board. 

John  thought  it  would  be  a  very  interesting  adventure 
for  him  to  go  on  board  in  this  way  in  a  small  boat,  and  for 
that  reason  was  inclined  to  take  the  Dover  route. 

But  then  he  did  not  know,  and  he  had  no  ready  means 
of  ascertaining,  whether  the  tide  would  be  low  or  not  at 
the  time  of  sailing  on  the  following  day  ;  and  if  it  should 
not  be,  he  saw  that  he  should  lose  his  desired  adventure, 
and  would  have  to  go  on  board  in  the  tame  and  common- 
place way  of  walking  over  a  plank  from  the  pier. 


146  FOLKESTONE. 

The  arrangements  of  the  Dover  and  Calais  line  are  mack 
with  special  reference  to  the  carrying  of  the  mails,  which, 
in  order  that  the  postal  service  between  London  and  Paris 
may  be  regular,  have  to  leave  at  the  same  hour  every  day, 
whatever  may  be  the  state  of  the  tide. 

But  the  communication  by  Folkestone  and  Boulogne  is 
conducted  more  particularly  with  a  view  to  the  conven- 
ience of  passengers.  Accordingly,  the  time  of  the  leaving 
of  the  boat  from  Folkestone  is  governed  by  the  state  of  the 
tide,  so  that  the  passengers  can  always  embark  on  the  En- 
glish side,  and  disembark  on  the  French  side,  directly  from 
the  pier.  This  makes  it  necessary  that  the  hour  of  depart- 
ure should  change  from  day  to  day,  according  to  the  state 
of  the  tide ;  and  John  found,  on  looking  at  the  time-table, 
that  the  hour  of  sailing  for  the  next  day  was  at  ten  o'clock 
in  the  evening.  Now  he  thought  that  going  on  board  a 
steamer  at  ten  o'clock  in  the  evening,  and  making  the  voy- 
age in  the  night,  would  be  more  romantic  than  even  going 
out  to  join  a  steamer  lying  in  the  open  roadstead  in  an 
open  boat,  and  so  he  decided  at  once  in  favor  of  Folke- 
stone and  Boulogne. 

He  rose  early  the  next  morning,  so  as  to  accomplish  one 
of  his  three  hours  of  study  before  breakfast.  He  would 
like  to  have  done  more  than  that,  but  he  had  not  time, 
and  after  breakfast  he  was  so  much  occupied  in  making 
arrangements  for  commencing  the  journey  that  he  could 
do  no  more  before  setting  out.  He  determined  on  taking 
the  first  train  after  breakfast. 

"You  see, by  that  plan,"  he  said  to  Lawrence,  "we  shall 
have  more  time  at  Folkestone ;  only  I  should  have  liked  to 
stay  here  a  little  longer,  to  do  some  more  of  my  studying." 

"  You  can  do  it  just  as  well  at  Folkestone,"  said  Law- 
rence. "There  is  an  excellent  place  to  write  and  study 
there." 


THE   READING-KOOM.  147 

"  What  sort  of  a  place  ?"  asked  John. 

"  The  reading-room  at  the  hotel  near  the  pier,"  said  Law- 
rence. "  There  is  a  charming  reading-room  there,  with  a 
very  large  round  table  in  the  centre  covered  with  maga- 
zines and  pictorial  papers,  and  pretty  desks  and  comforta- 
ble arm-chairs  at  the  windows  all  around  the  room.  There 
is  also  quite  a  large  library  of  books,  in  a  handsome  case, 
on  one  side." 

"That  will  be  just  the  place  for  me  to  study  my  other 
two  hours,"  said  John. 

This  reading-room,  which  is  really  a  very  attractive 
room — more  so  than  any  other  room  of  the  kind  that  I 
have  seen  in  any  hotel  in  England — is  a  very  appropriate 
and  desirable  part  of  the  accommodations  of  a  hotel  at  a 
sea-port,  where  people  are  liable  to  be  weather-bound,  so 
as  to  have  to  spend  some  hours,  and  perhaps  days,  in  wait- 
ing for  a  storm  to  abate  or  for  a  heavy  sea  to  go  down. 

Lawrence  and  John  arrived  at  Folkestone  about  morn- 
ing. The  town  is  situated  in  a  kind  of  dell,  opening  be- 
tween the  cliffs  of  the  coast.  This  dell  was  probably 
formed  in  the  course  of  ages  by  a  stream  of  water,  the 
mouth  of  which  has  been  deepened  and  enlarged  in  mod- 
ern times,  and  inclosed  in  long  piers  extending  out  from 
the  land  so  as  to  make  a  harbor.  Near  the  pier  on  one 
side  is  a  level  plateau,  which  is  laid  out  in  ornamental 
grounds,  in  the  centre  of  which  stands  the  hotel,  which  is 
called  the  Pavilion,  and  which  is  arranged  specially  for  the 
accommodation  of  the  travelers  departing  or  arriving  by 
the  steamers. 

The  first  thing  that  John  did  on  his  arrival,  after  giving 
the  porter  instructions  about  the  baggage,  or  the  luggage, 
as  they  always  call  it  in  England,  was  to  walk  a  little 
about  the  house  and  grounds  to  gratify  his  curiosity  by  ex- 
amining the  locality.  The  main  building  of  the  hotel  is 


148  FOLKESTONE. 

in  the  centre,  and  is  used  for  sitting-rooms  and  bedrooms, 
with  an  office  for  the  reception  of  guests  at  the  farther  end 
of  it.  The  principal  restaurant  is  in  another  building  facing 
this ;  the  entrance  to  this  restaurant  is  under  a  piazza.  In 
the  end  of  this  same  building,  toward  the  right,  is  situated 
the  reading-room  which  Lawrence  had  described,  and  on 
the  left  is  the  regular  dining-room.  The  pier  and  dock, 
where  the  steam-boats  lie,  is  upon  the  other  side  of  the 
street  from  the  hotel. 

John  explored  all  these  precincts  with  much  interest  and 
attention.  Among  other  things,  he  saw  a  placard  put  up 
signifying  that  there  was  a  public  dinner  given  every  day, 
or,  as  they  call  it,  a  table  d'hote,  toward  the  latter  part  of 
the  afternoon,  where  those  of  the  guests  of  the  hotel  who 
liked  to  do  so  could  dine  together.  There  was  also  the 
restaurant,  where  those  who  chose  could  dine  at  small  ta- 
bles by  themselves  at  any  hour  of  the  day. 

John  went  to  report  these  facts  to  Lawrence,  whom  he 
had  left  in  the  reading-room,  where  he  was  engaged  at  one 
of  the  desks  near  a  window  making  some  memoranda  in 
his  journal.  But  he  found  that  there  were  a  number  of  la- 
dies and  gentlemen  engaged  in  reading  and  writing  in  dif- 
ferent parts  of  the  room,  and  he  perceived  at  once  that  con- 
versation, except  in  the  lowest  whisper,  would  be  improper 
in  such  a  place ;  so  he  amused  himself  in  looking  at  some 
of  the  pictorial  papers  on  the  table  until  Lawrence  had  fin- 
ished his  work,  and  then  they  went  out  together. 

John  first  conducted  Lawrence  to  the  placard  which  con- 
tained the  notice  about  the  dinner,  and  asked  him  what  he 
should  do. 

"  Shall  we  dine  at  the  public  table,"  he  asked, "  or  have 
a  dinner  by  ourselves  ?" 

"  That  is  for  you  to  say,"  replied  Lawrence ;  "  I  have 
nothing  to  do  with  it.  You  are  taking  me  from  London 


PLANNING   FOB   THE    DAY.  149 

to  Paris,  and  all  the  arrangements  of  the  journey  are  un- 
der your  direction." 

"  Then,"  said  John,  "  I  shall  decide  to  have  dinner  at  the 
public  table,  so  that  I  can  see  the  people." 

"  All  right,"  said  Lawrence. 

"  Only,"  continued  John, "  I  do  not  know  whether  I  can 
wait  so  long.  The  dinner  is  not  till  six  o'clock." 

"  That's  bad,"  said  Lawrence. 

"  Let's  have  something  for  luncheon  !"  exclaimed  John, 
his  countenance  suddenly  brightening  up  as  if  a  sudden 
thought  had  struck  him. 

"I  think  that  is  what  I  should  do,"  said  Lawrence,  speak- 
ing in  a  somewhat  indifferent  tone,  as  if  the  affair  was  no 
concern  of  his.  "I  think,  if  I  had  charge  of  persons  on  the 
passage  from  London  to  Paris,  I  should  not  expect  to  let 
them  go  from  nine  o'clock  till  six  without  giving  them 
something  to  eat." 

"  We'll  have  a  nice  luncheon,"  said  John,  speaking  in  an 
exulting  tone.  "We  will  have  it  in  an  hour  from  this 
time.  I  am  going  to  study  my  second  hour  now,  at  once, 
before  I  go  out  to  see  any  thing  ;  then  we  will  have  lunch- 
eon, and  after  that  I  am  going  out  to  take  a  nvalk  and  see 
the  town ;  then,  after  dinner,  I  shall  have  time  for  another 
half  hour  of  study — from  half  past  seven  to  eight ;  and  if 
you  will  give  me  a  lecture  of  half  an  hour  on  board  the 
steamer  while  we  are  going  across,  I  shall  be  all  right." 

Lawrence  seemed  to  approve  of  this  plan — at  least  so 
John  thought,  and  it  was  accordingly  adopted.  John  went 
into  the  reading-room  and  established  himself  at  the  pleas- 
antest  looking  desk  which  he  saw  unoccupied,  and  prose- 
cuted his  work  diligently  for  an  hour,  while  Lawrence  sat 
in  a  very  comfortable  arm-chair  near  the  great  table,  and 
became  apparently  much  interested  in  reading  some  of  the 
reviews.  At  the  end  of  the  hour  they  went  together  into 


150  FOLKESTONE. 

the  restaurant,  where  they  had  an  excellent  luncheon.  Aft- 
er the  luncheon  they  set  out  for  a  walk.  John  was  full  of 
curiosity  to  see  the  pier  and  the  harbor,  and  also  the  town 
and  its  environs,  and  the  pleasure  which  he  enjoyed  in  the 
excursion  was  greatly  heightened  by  the  thought  that  so 
large  a  portion  of  his  work  was  done. 

"  I  thought  at  first,"  said  he, "  that  I  would  go  and  take 
the  walk  first,  and  afterward  have  my  hour's  study ;  but  I 
concluded  that  I  should  enjoy  the  walk  more  to  have  my 
second  hour  of  study  off  my  mind." 

"That  is  very  good  philosophy,  I  think,"  said  Lawrence. 

"  I  think  it  was  a  good  plan,"  replied  John, "  but  I  don't 
see  much  philosophy  in  it." 

"  The  philosophy  is  this,"  replied  Lawrence, "  that  if  you 
take  your  study  hour  first,  you  have  only  the  work  itself 
to  do,  and  the  enjoyment  of  the  ramble  afterward  is  with- 
out alloy ;  whereas  if  you  postpone  it,  you  have  the  work 
to  do  in  the  end  just  the  same,  and  the  irksomeness  of 
thinking  of  it  and  dreading  it  all  the  time  that  you  are 
taking  your  walk,  in  addition.  Thus  you  have  a  double 
burden  in  the  latter  case,  and  only  a  single  one  in  the 
former.  So  you  see  there  is  sound  philosophy  in  keeping 
well  ahead  with  your  work.  Duty  first,  and  pleasure  aft- 
erward, is  an  excellent  rule  in  respect  to  the  philosophy  of 
it,  as  well  as  on  other  accounts." 

The  two  boys,  or  the  two  young  men,  whichever  you 
think  is  the  most  appropriate  mode  of  designating  them, 
enjoyed  their  walk  very  highly.  They  went  first  out  to 
the  pier  which  formed  the  inclosure  of  the  harbor  on  the 
side  toward  the  hotel.  They  saw  a  great  number  of  steam- 
ers and  of  sailing  vessels  lying  in  the  harbor,  most  of  them 
aground  in  the  mud,  for  it  was  at  this  time  low  tide.  At 
the  outer  portion  of  the  pier  was  a  pretty  esplanade,  with 
seats  at  various  places,  and  a  light-house  near  the  end. 


THE    SCENE   ON  THE    CLIFFS.  151 

There  was  a  great  drawbridge  over  one  of  the  openings 
into  the  harbor,  made  for  the  passage  of  railway  trains  to 
a  great  station  on  the  pier  opposite  the  steamer  landing. 
After  examining  all  these  things  our  travelers  went  into 
the  town,  and  thence  up  a  long  ascending  road — with 
flights  of  stone  steps  branching  off  from  it  here  and  there 
— which  led  up  to  the  cliffs  on  one  side,  where  they  found 
long  rows  of  handsome  houses,  the  summer  residences  of 
fashionable  people  from  London.  There  were  lawns  and 
other  ornamental  grounds  in  front  of  these  houses,  between 
them  and  the  road ;  and  between  the  road  and  the  margin 
of  the  cliff  was  an  open  space,  serving  as  a  promenade. 
There  were  seats  here  and  there,  with  children  playing 
around  them,  while  those  who  had  charge  of  the  children 
were  sitting  upon  the  seats,  sewing  or  knitting.  In  other 
places  ladies  and  gentlemen  were  walking  to  and  fro,  en- 
joying the  magnificent  prospect  which  was  spread  before 
them  over  the  sea. 

This  handsome  esplanade,  with  the  sea  on  one  side  far 
below,  and  long  rows  of  elegant  houses  on  the  other,  ex- 
tended a  long  distance — a  mile  or  more — along  the  cliffs. 
In  one  place  John  went  near  enough  to  the  brink  of  the 
cliffs  to  look  over.  He  saw,  at  a  great  distance  below,  a 
road  running  along  close  to  the  margin  of  the  water,  with 
people  walking  upon  it,  and  here  and  there  a  cart  going 
along.  The  road  looked  very  narrow,  and  the  men  and  the 
carts  very  small. 

After  continuing  their  walk  until  their  curiosity  in  re- 
spect to  this  place  was  fully  satisfied,  Lawrence  and  John 
returned  to  the  town  in  the  valley  by  another  way,  on  the 
farther  side  of  the  range  of  houses  facing  the  sea. 

They  reached  the  hotel  an  hour  before  time  for  dinner, 
so  they  took  another  walk  around  the  harbor,  examining 
the  structure  of  the  piers,  and  looking  at  the  different 


152  .FOLKESTONE. 

steamers  and  vessels.  The  tide  was  coming  in,  and  some 
of  the  smaller  vessels  were  afloat.  They  found  the  steamer 
there  in  which  they  were  to  go  that  evening.  They  knew 
it  by  the  smoke  which  was  issuing  from  the  chimney,  which 
showed  they  wei-e  "  firing  up"  on  board,  so  as  to  have  the 
steam  ready  when  the  hour  should  arrive.  They  went  on 
board  this  steamer,  descending  by  a  long  and  steep  gang- 
plank, and  John  chose  the  place  where  he  said  that  he  and 
Lawrence  would  sit  during  the  passage. 

When  the  time  arrived  for  dinner,  they  went  to  the  ho- 
tel, which  was  very  near,  and  after  dinner  John  went  into 
the  reading-room,  and  spent  half  an  hour  at  his  studies. 
This  left  him  half  an  hour  more  of  work  to  complete  his 
task  for  the  day,  and  this  he  was  going  to  take,  as  has  al- 
ready been  stated,  in  the  form  of  a  conversational  lecture 
on  the  passage. 

Clouds  had  begun  to  gather  in  the  sky  before  Lawrence 
and  John  went  in  to  dinner,  and  when  John  had  finished 
his  study  he  found  that  the  sky  was  entirely  overcast,  and 
that  it  was  beginning  to  rain. 

"Lawrence,"  said  John,  after  going  to  the  door  and  look- 
ing out,  "it  is  beginning  to  rain.  There's  a  storm  coming 
on." 

"  Yes,"  said  Lawrence,  "  so  I  see." 

"  And  what  shall  we  do  about  going  ?" 

"Do  just  what  you  think  best,"  said  Lawrence;  "you 
are  in  command." 

"  But  I  can  ask  advice,  I  suppose,"  said  John.  "  What 
would  you  advise  me  to  do  ?" 

"Well,"  said  Lawrence,  "it  is  just  as  you  please;  but  if 
you  ask  my  advice,  I  should  not  advise  you  to  go  unless 
the  boat  goes." 

"  Nonsense,  Lawrence !"  said  John.  "  Of  course  we  can 
not  possibly  go  unless  the  boat  goes." 


AN  OLD  TRAVELER'S  RULE.  153 

"Not  very  well,"  said  Lawrence.  "You  might,  I  sup- 
pose, hire  a  sail-boat." 

Lawrence  said  this  with  a  perfectly  grave  face,  as  if 
he  thought  that  the  idea  "of  chartering  a  small  sloop  or 
schooner  for  crossing  the  Channel  by  themselves,  on  a 
night  and  in  a  storm  in  which  the  officers  of  a  steamer 
thought  it  not  safe  to  go  out,  was  one  to  be  seriously  en- 
tertained. 

"No ;  but  seriously,"  said  John, "  what  would  you  really 
do  if  you  were  I  ?" 

"I  don't  know  what  I  should  do  if  I  were  you,"  said 
Lawrence,  "but  in  such  cases — unless  I  have  ladies  under 
my  charge — my  rule  is,  if  the  boat  goes,  I  go,  and  not 
without." 

"  Then  m  go,"  said  John. 

Accordingly,  about  half  past  nine,  John  conducted  Law 
rence  on  board  in  the  rain. 

G2 


154  THE   CHANNEL   AT   NIGHT. 


CHAPTER  XVIII. 

THE    CHANNEL   AT   NIGHT. 

IT  was  dark  on  the  pier,  except  so  far  as  the  lamps  upon 
the  lamp-posts  enlightened  the  scene.  There  were  very  few 
passengers,  for  travelers  crossing  the  Channel  on  tours  of 
pleasure — which  class  of  persons  generally  constitute  the 
majority  on  board  these  steamers — usually  avoid  choosing 
their  time  for  crossing  when  the  tide  serves  at  night,  and 
some  on  this  occasion,  who  had  intended  to  go,  were  de- 
terred by  the  prospect  of  a  dark  and  rainy  passage,  and 
concluded  to  remain  quietly  in  the  comfortable  hotel  till 
the  next  day. 

It  was  wet  upon  deck,  and  so  Lawrence  and  John  went 
below.  There  they  found  a  small  cabin,  with  tables  in  the 
middle  of  it,  and  seats  along  the  sides.  They  chose  a  place 
in  a  snug  corner,  where  they  sat  for  a  while  amusing  them- 
selves with  watching  the  coming  and  going  of  the  people. 
One  man  came  in  seeming  very  much  out  of  humor,  and 
uttering  very  impatient  expressions  about  the  weather; 
and  then,  after  putting  doAvn  his  valise  and  his  parcels, 
and  looking  about  with  an  angry  air,  he  stalked  out  again. 

"He  is  grumbling  about  the  weather,"  said  John. 

"  Yes,"  said  Lawrence ;  "  he  does  not  seem  to  be  aware 
that  complaining  of  the  weather  is  complaining  of  the  prov- 
idence of  God." 

"  I  don't  think  that  the  weather  is  more  the  providence 
of  God  than  any  thing  else,"  said  John.  "It  is  all  accord- 
ing to  the  laws  of  nature." 

John  had  read  this  in  .some  book,  and  it  is  no  doubt 
correct. 


CROSSING   IN   THICK    WEATHER.  155 

"  True,"  replied  Lawrence.  "  The  weather  is  no  more 
controlled  by  the  providence  of  God  than  every  thing 
else ;  and  so,  when  a  man  grumbles  and  complains  about 
any  thing  that  he  can  not  help,  he  is  repining  against  the 
providence  of  God,  and  he  punishes  himself  in  doing  it." 

"  How  so  ?"  asked  John. 

"  He  makes  himself  uncomfortable  in  getting  angry,  and 
does  not  help  the  trouble  by  it." 

"  But  sometimes  he  can  help  it,"  replied  John. 

"I  said  things  that  he  could  not  help,"  rejoined  Law* 
rence.  "  If  there  is  any  thing  that  he  can  help,  he  ought 
to  help  it." 

Just  then  the  engine  began  to  move,  and  John  said  the 
steamer  was  going  to  start,  and  that  he  must  go  up  on  deck 
to  see  her  sail  out  of  the  harbor.  In  about  ten  minutes  he 
came  back. 

"It  does  not  rain  any  more,"  he  said,  "but  it  is  very  wet 
on  deck,  and  the  light -house  behind  us  on  the  pier  looks 
very  brilliant.  I'm  glad  we  came." 

"  It  is  generally  best  to  go  if  the  boat  goes,"  said  Law- 
rence ;  "  that  is,  if  you  are  a  man." 

"  I'm  not  a  man,"  said  John. 

"  Or  if  you  are  old  enough  to  have  much  manliness 
about  you,"  added  Lawrence. 

"I'm  going  up  again,"  said  John,  after  a  moment's  pause. 

So  he  left  Lawrence,  who  was  at  this  time  sitting  quietly 
at  a  table  reading.  He  was  gone  about  fifteen  minutes, 
and  when  he  came  back  he  said  that  the  light-house  on  the 
French  coast  was  in  view.  "  It  looks  like  a  faint  star,"  he 
said,  "  low  in  the  horizon." 

"  Then  the  clouds  must  have  lifted,  and  the  air  must 
have  become  quite  clear  below,"  replied  Lawrence,  "for 
very  rare  drops  of  rain,  or  even  a  very  little  dimness,  ex- 
tending for  twenty  miles,  would  entirely  intercept  the 


156  THE    CHANNEL    AT   NIGHT. 

light.  Each  drop  or  each  vesicle  would  intercept  and  ab- 
sorb a  part,  and,  in  encountering  the  immense  number  that 
would  be  contained  in  a  space  of  twenty  miles,  the  whole 
would  be  expended." 

Lawrence  then  said  that  he  had  a  plan  to  propose  about 
John's  studies  for  the  next  day. 

"To-morrow  will  be  rather  a  hard  day  for  you,"  he  said, 
"  in  respect  to  your  three  hours'  study.  You  will  be  up 
late  to-night,  and  so  will  not  be  much  inclined  to  rise  early 
in  the  morning.  Then  we  are  going  to  Paris  to-morrow, 
and  the  journey  by  rail  will  take  up  a  large  part  of  the 
day.  Finally,  the  excitement  of  arriving  in  Paris,  and  get- 
ting established  in  a  new  and  strange  hotel,  will  make  it 
hard  for  you  to  sit  down  to  study  then." 

"  But  I  mean  to  do  it,  nevertheless,"  said  John. 

"  You  can  give  it  up  for  to-morrow,  you  know",  if  you 
please,"  continued  Lawrence;  "there  is  no  penalty — only 
you  lose  a  little  credit." 

"That  is  just  what  I  don't  wish  to  do,"  said  John. 

"  Or  you  can  take  it  for  one  of  your  days  of  failure," 
added  Lawrence.  "  You  are  entitled  to  one  day  in  each 
fortnight." 

"No,"  said  John;  "I  am  not  going  to  have  any  days 
of  failure,  if  I  can  possibly  help  it.  I  have  not  had  any 
yet." 

"  Then  I'll  tell  you  what  I  propose,"  said  Lawrence.  "  I 
will  give  you  a  lecture  on  the  philosophy  of  bright  lights 
now,  while  we  are  making  the  passage ;  then  you  can 
write  an  abstract  or  recapitulation  of  it  when  we  get  on 
shore.  Time  passes  quicker  when  we  are  writing  than  in 
any  other  kind  of  study.  My  giving  you  a  lecture  of  half 
an  hour  will  finish  your  study  for  to-day ;  then,  if  you 
choose,  after  we  reach  Boulogne,  you  can  begin  to  write 
your  recapitulation  and  do  half  an  hour  of  to-morrow's 


THE    EVENING    LESSON.  157 

Btudy  to-night;  then  you  can  do  another  half  hour's  to- 
morrow morning  after  breakfast,  before  we  set  out  for 
Paris  ;  then  one  hour  of  study  on  the  journey,  and  another 
of  writing,  perhaps,  after  you  get  to  Paris,  will  make  up 
your  time." 

John  agreed  to  this  proposal,  and  then  they  both  went 
to  a  corner  of  the  cabin,  where  they  could  sit  together  and 
talk  by  themselves,  and  Lawrence  commenced  his  lecture 
at  once. 

"  The  first  thing  that  you  are  to  put  down  in  your  reca- 
pitulation," said  Lawrence,  "  is  that  the  general  principle 
on  which  bright  artificial  lights  are  produced  is  by  raising 
solid  substances  of  some  kind  to  an  intense  degree  of  in- 
candescence by  means  of  extreme  heat.  The  heat  is  gen- 
erally produced  by  combustion,  though  not  always  so. 
But,  however  the  heat  may  be  pi'oduced,  it  is  always  by  its 
effect  on  the  particles  of  a  solid  substance  in  causing  them 
to  emit  a  very  vivid  light  that  almost  all  bright  lights  are 
made. 

"Then,  the  second  thing  that  you  must  say,"  continued 
Lawrence, "  is  that,  in  order  to  produce  the  required  inten- 
sity of  heat  by  ordinary  combustion,  the  thing  to  be  done 
is  to  increase  the  supply  of  oxygen.  This  is  effected,  as 
you  know,  in  the  case  of  the  Argand  burner,  by  making 
the  wick  circular,  and  bringing  a  current  of  air  up  on  the 
inside  of  it,  and  also  increasing  the  strength  of  the  current 
on  the  outside  by  means  of  a  glass  chimney. 

"  So,  after  stating  the  two  principles,  namely,  first,  that 
the  light  is  produced  by  making  solid  particles  intensely 
incandescent  by  means  of  extreme  heat,  and,  secondly,  that, 
when  this  heat  is  to  be  produced  by  combustion,  it  is  done 
by  increasing  in  some  way  the  supply  of  oxygen,  then  you 
can  enumerate  the  five  principal  modes  of  producing  bright 
light,  and  make  them  subordinate  heads  in  your  article, 


158  THE    CHANNEL   AT   NIGHT. 


INTERNAL    SUPPLY   OP  AIK. 

and  proceed  to  explain  them  in  order,  as  I  will  now  go  on 
to  explain  them  to  you. 

"  The  five  principal  kinds  of  bright  light  produced  by 
artificial  means  are, 

"  1.  The  Argand  burner. 

"  2.  The  Bude  Light. 

"  3.  The  Oxyhydrogen  Light. 

"  4.  The  Magnesium  Light. 

"  5.  The  Electric  Light. 

"1.  THE  ARGAND  BURNER.  This  I  have  already  ex- 
plained," continued  Lawrence.  "  The  solid  made  luminous 


THE    BUDE    AND    OXYHYDROGEN   LIGHTS.  159 

by  heat  consists  of  particles  of  carbon,  heated  by  the  com- 
bustion chiefly  of  hydrogen  from  the  oil,  and  increased  in 
intensity  by  currents  of  air  both  on  the  inside  and  outside 
of  the  flame. 

"  2.  THE  BUDE  LIGHT  is  the  same,  except  that,  instead 
of  currents  of  air,  currents  of  pure  oxygen  are  made  to  flow 
in  contact  with  the  flame.  Air  contains  only  about  one 
fifth  of  its  bulk  of  oxygen ;  the  rest  is  nitrogen,  which,  for 
the  purposes  of  combustion,  is  only  in  the  way.  The  ra- 
pidity and  intensity  of  the  combustion  is  greatly  increased 
by  supplying  pure  oxygen  to  the  fire.  In  the  burning  of 
magnesium,  it  is  found,  by  careful  comparison,  that  the  in- 
tensity of  the  light  is  doubled  when  it  is  supplied  with 
pure  oxygen.  In  the  same  way,  the  light  in  the  Argand 
burner  is  vastly  increased  when  pure  oxygen  is  supplied 
to  it  instead  of  common  air. 

"  The  Bude  light  has  been  used  a  great  deal,  sometimes 
for  light-houses,  and  sometimes  for  other  purposes;  but 
there  was  found  to  be  this  inconvenience  about  it,  that  it 
required  the  apparatus  for  producing  the  oxygen  always 
at  hand ;  and  the  process  required  a  good  deal  of  atten- 
tion, and  involved  some  increased  expense. 

"  3.  THE  OXYHYDROGEN  LIGHT.  In  the  case  of  the  Ar- 
gand and  Bude  lights,  the  solid  substance  rendered  incan- 
descent is  composed  of  particles  of  carbon,  which  are  fur- 
nished, together  with  the  hydrogen  for  producing  the  heat, 
by  the  oil  or  other  hydrocarbon  that  is  burned.  But  in 
the  oxyhydrogen  light,  the  hydrogen,  as  well  as  the  oxy- 
gen, is  furnished  pure,  and  the  heat  which  is  produced  by 
their  combustion  is  directed  against  the  point  of  a  cone 
formed  of  lime,  or  some  other  substance  capable  of  sustain- 
ing such  a  heat  without  melting  or  burning,  and  this  lime 
is  the  solid  substance  which  becomes  incandescent  and 
emits  the  light." 


160  THE    CHANNEL    AT   NIGHT. 

"The  heat  produced  by  the  burning  in  this  case — that  is, 
the  chemical  union  of  pure  hydrogen  and  oxygen,  is  incon- 
ceivably intense.  There  is  scarcely  any  substance  so  re- 
fractory as  to  endure  it.  Any  substance,  however,  that  can 
sustain  it,  is  raised  to  such  an  intense  incandescence  by  the 
heat  that  it  emits  a  light  of  the  most  wonderful  power.  It 
has  been  seen,  it  is  said,  at  the  distance  of  more  than  one 
hundred  miles  in  the  daytime. 

"  The  light  produced  thus  by  an  oxyhydrogen  flame  pro- 
jected upon  lime  is  sometimes  called,  from  the  source  of 
it,  the  oxyhydrogen  light,  and  sometimes  the  lime  or  cal- 
cium light,  and  sometimes  the  Drummond  light,  from  the 
name  of  the  man  who  first  discovered  it,  or,  at  least,  who 
first  introduced  it. 

"There  is  one  curious  circumstance  in  connection  with 
this  subject,"  continued  Lawrence, "  which  you  can  men- 
tion in  your  recapitulation  or  not,  just  as  you  think  best, 
after  you  learn  what  it  is,  and  that  is,  that  it  makes  appar- 
ently no  difference  what  the  solid  substance  is  which  is 
raised  to  this  intense  incandescence  in  the  various  methods 
adopted  of  heating  them.  The  particles  of  carbon  in  the 
Bude  light,  for  example,  are  black  when  they  are  cold,  but 
they  give  out  a  none  the  less  intense  light  on  that  account 
when  they  are  heated  up  to  the  requisite  point.  Indeed, 
it  is  very  curious  that  all  solid  substances,  however  differ- 
ent they  may  be  in  chemical  or  mechanical  properties,  be- 
gin to  become  luminous  at  the  same  temperature,  and  are, 
so  far  as  I  know,  equally  brilliant  at  the  highest  tempera- 
tures. The  reason,  therefore,  for  using  lime  is  not  because 
it  is  white,  but  because  it  will  stand  the  heat  without  melt- 
ing. 

"But  you  must  remember  that  while  all  solid  substances 
become  incandescent  at  the  same  temperature,  and  emit,  as 
I  suppose,  the  same  quantity  of  light  in  comparison  with 


MAGNESIUM   AND   ELECTRIC   LIGHTS.  161 

each  other  at  all  temperatures,  there  is  a  vast  difference  in 
this  respect  between  solids  and  gases.  Gases,  however 
highly  they  are  heated,  for  some  mysterious  reason  or 
other,  emit  comparatively  very  little  light.  There  is  a  curi- 
ous experiment  to  show  this.  If  a  thin  plate  of  platinum 
is  held  over  the  flame  of  a  lamp  at  a  place  where  the  as- 
cending gases  are  not  at  all  luminous,  it  becomes  incandes- 
cent itself  at  once — that  is,  a  degree  of  heat  which  makes 
the  solid  emit  a  bright  light,  will  not  cause  the  gas  to  emit 
any  at  all." 

"  Would  it  be  the  same  with  a  thin  piece  of  iron  or 
steel,"  asked  John — "  a  piece  of  watch-spring,  for  exam- 
ple?" 

"  I  don't  know,"  replied  Lawrence.  "  Perhaps  it  might 
over  an  Argand  lamp,  or  any  lamp  on  that  principle,  with 
a  glass  chimney." 

"I  mean  to  try  it  some  day,"  said  John. 

"I  would  do  so,"  said  Lawrence.  "And  now  for  the 
fourth  light,  which  is 

"  4.  THE  MAGNESIUM  LIGHT." 

But  what  Lawrence  said  on  this  head  need  not  be  re- 
peated, as  the  manner  in  which  a  very  bright  light  is  pro- 
duced by  the  combustion  of  magnesium,  and  by  the  intense 
incandescence  of  the  solid  particles  of  magnesia  which  re- 
sult, has  already  been  fully  explained. 

"And  now,"  continued  Lawrence,  after  having  finished 
what  he  had  to  say  under  the  fourth  head, "we  come  to 

"  5.  THE  ELECTRIC  LIGHT. 

"  This  light  is  on  the  same  principle  with  the  others  in 
respect  to  its  being  produced  through  the  incandescence 
of  solid  particles  by  intense  heat,  and  the  particles,  too,  are 
particles  of  carbon ;  but  the  heat  is  produced  in  another 
way,  and  that  is  not  by  any  process  of  combustion,  but  by 
electricity. 


162  THE    CHANNEL   AT    NIGHT. 

"  But  I  think,"  said  Lawrence, "  that  I  have  given  you  a 
long  enough  lecture  for  this  time,  so  I  will  leave  the  elec- 
tric light  for  to-morrow,  when  we  are  going  on  in  the  train 
to  Paris." 

"Yes,"  replied  John,  "that  will  be  better;  you  have  told 
me  now  as  much  as  I  can  well  remember.  I  think  I  had 
better  make  a  memorandum  with  my  pencil  of  the  three 
first  kinds  of  light,  and  that  will  help  me  in  writing  my 
article." 

So  John  took  his  note-book  from  his  pocket  and  went  to 
the  table  to  make  his  memorandum. 

The  time  occupied  by  the  lecture  was  more  than  half  an 
hour,  as  a  great  many  things  were  said  which  are  not  re- 
corded in  my  report  of  the  conversation.  Accordingly, 
when  John,  after  completing  his  memorandum,  went  up  on 
deck,  he  found  that  the  light  which  was  beaming  from  the 
harbor  at  Boulogne  was  a  great  deal  brighter  and  seemed 
much  nearer.  Indeed,  they  were  now  about  half  across 
the  Channel.  They  arrived  about  midnight,  and  John,  find- 
ing, when  they  reached  the  hotel,  that  he  was  too  tired  and 
sleepy  to  write,  concluded  to  go  to  bed,  without  attempting 
to  do  any  of  the  next  day's  work  that  night,  which  Law- 
rence thought  was  a  very  wise  conclusion. 

After  he  had  gone  to  bed,  and  just  as  he  was  going  to 
sleep,  he  called  out  through  the  open  door  to  Lawrence, 
who  was  in  the  next  room, 

"  Lawrence,  I  forgot  all  about  my  plan  of  going  to  Paris 
second  class !" 

"  Never  mind,"  said  Lawrence ;  "  lie  down  and  go  to 


THE    ELECTRIC   LIGHT.  163 


CHAPTER  XIX. 

THE   ELECTRIC   LIGHT. 

THE  four  very  bright  lights  which  can  be  produced  by 
artificial  means  are,  as  Lawrence  enumerated  them  in  his 
explanations  to  John  (if  we  leave  out  the  Argand  burner, 
which  is,  after  all,  only  the  form  of  a  burner,  and  not  a 
special  mode  of  producing  light),  the  Bude  Light,  the 
Magnesium  Light,  the  Oxyhydrogen  Light,  and  the  Elec- 
tric Light.  The  last  named — the  Electric  Light — is  in 
some  respects  the  most  remarkable  of  all. 

The  electric  light  is  like  the  others  which  have  been  de- 
scribed in  this  respect,  namely,  that  it  acts  on  the  general 
principle  of  raising  solid  particles  to  an  intense  degree  of 
incandescence  by  means  of  extreme  heat,  while  it  differs 
from  them  all  in  the  manner  in  which  the  heat  is  pro- 
duced. In  the  other  three  the  heat  is  produced  directly 
by  the  process  of  combustion,  which,  as  we  have  already 
seen,  is  another  name  for  the  force  which  is  developed  by 
the  combination  of  the  combustible — chiefly  hydrogen — 
with  oxygen.  In  this,  on  the  other  hand,  the  heat  is  pro- 
duced by  a  current  of  electricity,  though  it  is  a  remarka- 
ble instance  of  the  analogy  which  runs  through  the  opera- 
tions of  nature,  that  the  current  of  electricity  which  devel- 
ops the  heat  is  often  produced  by  the  combustion  of  zinc, 
or,  rather,  by  a  process  which  is,  in  a  chemical  sense,  essen- 
tially combustion.  The  current,  nevertheless,  may  be,  and 
now  often  is,  produced  in  other  ways. 

In  whatever  way  the  movement  of  electricity  is  occa- 
sioned, it  often  produces  luminous  effects.  The  lightning 


164  THE    ELECTRIC   LIGHT. 

in  the  clouds  is  the  most  striking  of  these  effects  that  is 
witnessed  in  nature.  The  aurora  borealis  is  another  of  the 
forms  in  which  electrical  light  is  manifested.  In  certain 
states  of  the  atmosphere,  also,  pencils  of  light  are  seen 
upon  pointed  objects,  such  as  the  tips  of  the  masts  and 
spars  of  a  vessel  at  sea,  and  the  summits  of  spires  and  oth- 
er projecting  points  of  buildings  on  land. 

There  is  a  good  deal  of  mystery  about  some  of  the  forms 
in  which  the  light  produced  by  electricity  appears,  but  in 
that  which  is  developed  by  artificial  means  for  purposes 
of  illumination,  it  is  well  ascertained  that  the  effect  is  due 
to  the  incandescence  of  solid  particles  by  the  agency  of  in- 
tense heat.  It  is  found  that,  when  an  interruption  is  made 
in  an  electrical  circuit,  heat  is  developed,  provided  that  the 
current  is  powerful  enough  to  force  its  way  across  the  in- 
terval. Sometimes  the  interruption  consists  of  a  wire  or 
other  conductor  too  slender  to  convey  the  whole  current. 
In  this  case  the  wire  is  at  once  heated  more  or  less  intense- 
ly, according  to  the  force  of  the  current  in  relation  to  the 
slenderness  of  the  wire. 

This  is  the  way,  in  fact,  in  which  blasting  charges,  for 
example,  are  often  fired  in  rocks  or  under  water.  The  car- 
tridge containing  the  charge  is  prepared  beforehand  by  two 
wires  coming  in  on  opposite  sides  of  it,  and  connected  to- 
gether in  the  middle  of  the  gunpowder  by  a  very  slender 
tcire,  too  fine  to  convey  readily  the  whole  electrical  charge 
which  is  to  pass  through  the  circuit.  The  outer  ends  of 
the  two  side  wires  are  then  connected  with  the  electrical 
battery  in  such  a  manner  that  the  charge  may  at  any  mo- 
ment be  sent  through  them.  The  battery,  of  course,  may 
be  placed  at  any  distance,  provided  that  wires  can  be  laid, 
or  other  electrical  communications  made  from  the  two  poles 
of  it  to  the  place  where  the  cartridge  is  deposited.  Then, 
\vhen  the  battery  is  set  in  operation,  and  the  connections 


EI.ECTK1FIED  POINTS. 


CHABOOAL   rol.NTU — ilAG.NI 


SOURCE    OF   THE    ILLUMINATION.  169 

are  properly  joined,  the  fine  wire  within  the  cartridge  is 
instantaneously  ignited,  and  the  gunpowder  or  other  ex- 
plosive material  is  fired. 

When  the  interruption  in  the  circuit  is  made  under  cer- 
tain circumstances  and  iu  a  certain  way,  the  intervening 
space  is  filled  with  extremely  minute  particles,  which  are 
detached  from  the  solid  substance  at  one  side  of  the  inter- 
val and  driven  across  to  the  other  side  in  a  state  of  intense 
heat  and  incandescence.  This  especially  takes  place  when 
the  two  terminations  on  each  side  of  the  interval  are  formed 
of  cones  of  carbon.  In  this  case  particles  of  the  carbon,  so 
minute  as  to  be  individually  entirely  invisible,  become  de- 
tached from  one  side  and  pass  across  through  the  air  to  the 
other  in  a  state  of  incandescence  so  intense  as  to  furnish  a 
light  which  surpasses  almost  every  other  artificial  light  in 
brilliancy. 

And  this  is  the  famous  electrical  light. 

The  charcoal  points  used  are  actually  very  small.  The 
engraving,  however,  shows  the  effect  which  is  produced, 
and  the  result  of  it  in  modifying  the  forms  of  the  points, 
as  seen  greatly  magnified.  The  globules  of  melted  matter 
which  appear  attached  to  the  cones  come  from  the  fusion 
of  the  earthy  impurities  in  the  carbon. 

Although  the  light  is  thus  derived  from  incandescent 
particles  of  carbon,  it  is  not  at  all  due  to  heat  produced  by 
the  combustion  of  them,  as  is,  in  a  great  measure,  the  case 
with  the  light  which  comes  from  a  common  fire — that  is 
to  say,  the  heat  which  renders  them  incandescent  is  not 
the  heat  derived  from  their  own  combination  with  oxygen, 
but  from  that  developed  by  the  electricity  alone,  which  is 
vastly  more  intense  than  any  heat  which  their  combustion 
would  produce.  The  evidence  of  this  is  that  the  electric 
light  is  equally  vivid  in  a  vessel  exhausted  of  aii\  as  shown 
in  the  engraving  on  the  following  page. 
H 


170  THE    ELECTRIC   LIGHT. 

Above  and  below  we  see  the  two  wires  conducting  the 
electric  current,  and  connected  re- 
spectively with  the  charcoal  points 
within  the  egg-shaped  glass.  This 
glass  is  closed  above  around  the 
metallic  rod  passing  down  through 
the  cap  of  it.  There  is  an  opening 
in  the  upper  end  of  this  rod,  into 
which  the  wire  from  the  battery 
can  be  inserted  and  secured  by  the 
thumb-screw,  which,  however,  is  so 
small  in  the  engraving  as  to  be 
scarcely  visible.  There  is  a  simi- 
lar connection  below  with  the  oth- 
er battery  wire. 
There  is  an  opening  through  the  stem  and  the  base  of 
the  instrument  beloAV,  by  means  of  which  the  air  may  be 
exhausted — the  instrument  being  placed  upon  the  plate  of 
an  air-pump  for  this  purpose — arid  then  the  opening  can  be 
closed  by  means  of  the  stop-cock,  the  thumb-piece  of  which 
is  seen  in  its  proper  place  on  the  left-hand  side  of  the  stem. 
With  this  instrument  it  is  shown  that  the  vividness  of 
the  light  is  not  diminished  by  the  absence  of  air,  and,  con- 
sequently, that  the  source  of  the  heat,  by  which  the  parti- 
cles of  carbon  are  made  incandescent,  is  not  combustion, 
but  some  mysterious  property  of  the  current  of  electricity 
to  manifest  itself  under  certain  circumstances  in  that  form. 
All  these  things  about  the  different  modes  of  illumina- 
tion, as  used  in  light-houses,  and  a  great  deal  more  about 
the  application  and  use  of  them,  Lawrence  explained  to 
John  in  the  lecture  which  he  gave  him  in  the  railway  car- 
riage, on  their  way  to  Paris,  on  the  day  after  they  crossed 
the  Channel,  as  described  in  the  last  chapter.  The  lecture 
was  in  two  parts,  of  half  an  hour  each,  with  an  interval  of 


THE    CHANNEL   LIGHTS.  171 

two  or  three  stations  between.  During  this  interval  the 
train  stopped  at  Amiens  for  the  passengers  to  take  dinner. 
John  had  worked  for  an  hour  at  the  hotel  at  Boulogne  be- 
fore going  to  the  station,  and  he  intended  to  spend  an  hour 
in  Avriting  an  abstract  of  Lawrence's  lecture  when  he  should 
arrive  in  Paris ;  this  would  make  his  three  hours'  study 
for  that  day. 

"  If  I  had  known  all  this  about  the  different  kinds  of 
light  for  light-houses  before,"  said  John,  after  the  lecture 
Avas  concluded,  "I  should  have  changed  my  plan  about 
going  to  Paris  to-day." 

"  What  should  you  have  done  ?"  asked  Lawrence. 

"Instead  of  going  to  Paris,  I  should  have  gone  down  the 
coast  of  France,  bordering  on  the  Channel,  to  see  the  light- 
houses. We  are  in  the  best  place  to  see  light-houses  in 
the  whole  .world." 

"  Yes,"  said  Lawrence ;  "  the  English  Channel  is  admira- 
bly well  lighted,  on  both  the  French  and  English  sides." 

"  There  are  light-houses  on  every  point  of  land  and  at 
the  entrance  of  every  little  harbor,"  said  John;  "I  saw  a 
map  of  them.  I  suppose  there  must  be  a  great  many  dif- 
ferent kinds  that  we  might  have  seen  if  I  had  only  thought 
of  it." 

"It  would  not  have  done  any  good  for  you  to  have 
thought  of  it,"  said  Lawrence, "  for  you  have  no  authority 
to  decide  that  we  would  go  to  see  them." 

"Why  not?"  asked  John,  "I'm  commander  of  this  ex- 
pedition." 

"Yes,"  rejoined  Lawrence, "  but  with  limited  powers; 
you  ai-e  in  command  for  the  purpose  of  conducting  the 
party  to  Paris.  You  have  a  right  to  decide  upon  any 
course  and  any  mode  of  traveling  which  you  honestly  think 
best  adapted  to  take  us  to  Paris  in  an  agreeable  and  com- 
fortable manner,  but  you  have  no  authority  to  deviate  en- 


172  THE    ELECTRIC    LIGHT. 

tirely  from  the  object  for  which  you  were  appointed,  and 
take  us  somewhere  else. 

"Every  body  that  is  intrusted  with  power,"  continued 
Lawrence, "  is  bound  always  to  keep  in  mind  the  object, 
and  the  limits  of  it.  For  example,  if  the  Secretary  of  the 
Navy,  who  has  command  of  all  the  government  ships,  were 
to  fit  out  a  squadron  to  convey  a  party  of  his  private 
friends  on  an  excursion  of  pleasure  up  the  Mediterranean, 
he  would  entirely  transgress  the  limits  of  his  authority. 
His  power  over  the  navy  is  given  to  him  for  a  certain  ex- 
press purpose,  namely,  to  serve  the  interests  of  the  nation 
in  carrying  into  effect  the  policy  and  the  measures  deter- 
mined upon  by  Congress,  and  his  authority  is  bounded 
strictly  by  these  limits.  If  he  were  to  attempt  to  use  the 
navy  for  any  other  purpose  or  in  any  other  way,  he  would 
be  impeached  and  turned  out  of  office.  There  is  special 
provision  made  for  such  cases  by  the  government." 

"  How  ?"  asked  John. 

"If  any  public  officer  transgresses  the  limits  of  his  pow- 
er," said  Lawrence, "  the  House  of  Representatives  bring 
the  accusation  against  him,  and  the  Senate  try  him.  If  he 
is  proved  guilty,  he  is  summarily  ejected  from  office." 

"  But  there  isn't  any  government  in  our  case,"  said  John. 

"Yes,"  rejoined  Lawrence, "I'm  the  government — that 
is,  I  am  the  supreme  authority  in  all  this  tour ;  and  if  you 
had  decided  to  turn  off  from  the  route  to  Paris  to  go  down 
the  Channel  in  order  to  visit  the  light-houses,  I  should  have 
impeached  you  and  turned  you  out  of  office  forthwith." 

John  laughed  aloud  at  this  idea,  and  said  that  he  almost 
wished  that  he  had  done  it  for  the  sake  of  the  fun  of  being 
impeached  and  turned,  out  of  office. 

"Besides,"  continued  Lawrence,  smiling  a  little,  in  sym- 
pathy with  John's  amusement, "  you  want  to  know  a  good 
deal  more  about  lights  and  light-houses  yet  before  you  can 


ARRIVAL   I*T   PARIS.  172 

risit  them  profitably.  It  is  always  best  to  know  some- 
thing about  the  nature  and  character  of  any  contrivance 
before  you  go  to  examine  it.  If  we  understand  the  general 
plan  of  a  machine,  for  example,  and  the  principles  on  which 
it  operates,  before  we  see  it,  we  can  then  examine  it  intelli- 
gently. Things  have  a  significance  to  our  minds  in  that 
case  which  would  be  otherwise  wholly  unmeaning.  But  if 
we  go  to  look,  for  example,  at  the  arrangements  and  con- 
trivances at  a  first-class  light-house,  without  knowing  any 
thing  about  the  principles  which  govern  the  operation,  we 
can  only  stare  at  them  in  bewilderment  and  wonder,  and 
go  away  nearly  as  ignorant  as  we  came." 

John  was  convinced  that  this  was  true,  and  he  deter- 
mined that  on  his  arrival  in  Paris  he  would  first  write  a 
pretty  full  abstract  of  what  Lawrence  had  taught  him  in 
his  lecture,  and  then  he  would  procure  a  book  about  light- 
houses, and  learn  all  that  he  could  in  regard  to  the  differ- 
ent systems  adopted,  and  the  manner  in  which  the  arrange- 
ments are  carried  out,  especially  those  relating  to  light- 
houses along  the  coast  of  the  English  Channel. 

"  And  then,"  said  Lawrence, "  when  we  set  out  on  our 
return  from  Paris,  if  you  propose  that  we  should  go  down 
from  Boulogne  on  the  French  side,  visiting  the  principal 
light-houses  on  the  way,  and,  when  we  get  to  Havre,  cross 
over  and  come  up  to  Folkestone  on  the  English  side,  I 
shall  think  it  an  excellent  plan." 

On  the  arrival  of  the  train  at  Paris,  John  was  greatly  ex- 
cited at  the  spectacle  presented  to  his  mind  in  the  life  and 
movement  of  the  great  city,  for  this  journey  was  before  the 
desolation  and  ruin  brought  upon  it  by  the  great  Prussian 
war.  They  took  a  cab,  and  went  directly  to  the  Grand 
Hotel.  From  the  windows  of  the  cab  John  observed,  with 
great  interest  and  much  excitement,  the  wonderful  sights 
presented  to  his  view. 


174  THE    ELECTRIC    LIGHT. 

When  they  arrived  at  the  hotel,  arid  had  gone  through 
the  preliminary  ceremony  of  entering  their  names  and  en- 
gaging lodgings,  they  were  shown  to  their  rooms,  and  there 
the  first  thing  that  John  did  was  to  draw  up  a  table  near 
a  window  and  take  out  his  writing  materials. 

"  The  first  tiling  that  I  am  going  to  do,"  said  he, "  is  to 
get  my  hour  of  study  oiF  my  mind,  then  I  can  go  out  and 
see  the  city  entirely  at  my  ease." 

This  he  did.  While  he  was  thus  engaged,  Lawrence 
went  down  and  waited  for  him  in  the  splendid  reading- 
room  below.  The  reading-room  was  much  larger  and  more 
magnificent,  but  not  so  cosy  and  snug  as  the  one  at  Folke- 
stone. At  the  end  of  the  hour  John  came  down,  and  he 
and  Lawrence  went  out  into  the  grand  court-yard,  and 
thence  by  an  elegant  passage,  with  a  roadway  in  the  mid- 
dle, and  a  sidewalk  separated  from  the  roadway  by  col- 
umns on  each  side,  into  the  street.  John  almost  leaped  for 
joy  at  the  sight  of  the  scene  of  gayety  and  splendor  which 
here  met  his  eye. 

"  Lawrence,"  said  he,  "  I'm  glad  my  study  is  done,  and  I 
think  your  rule  of '  duty  first  and  pleasure  afterward '  is 
excellent  philosophy." 


LIGHTS   FOR   A   LIGHT-HOUSE.  175 


CHAPTER  XX. 

THE    CORRELATION   OF   FORCE. 

THE  work  of  establishing  a  light-house  upon  the  sea- 
coast  for  the  guidance  of  mariners  naturally  divides  itself 
into  two  portions,  or,  rather,  there  are  two  distinct  ends  to 
be  secured,  each  of  which  is  essential  to  success.  The  first 
is  to  devise  some  method  of  making  a  very  bright  light,  and 
the  second  the  means  of  gathering  the  beams  that  would 
naturally  radiate  backward  over  the  land,  or  upward  into 
the  sky,  and  throwing  them  all  forward  over  the  sea,  so 
that  they  may  be  brought  to  combine  their  luminous  effect 
in  the  direction  where  the  light  is  required. 

In  respect  to  the  former  point — that  is,  the  source  of 
light  itself — there  are  many  things  to  be  considered  be- 
sides the  actual  brightness  of  it.  The  concentration  of 
the  radiant  point  is  very  important,  inasmuch  as  light  is- 
suing from  a  point  is  much  more  manageable  by  lenses 
and  reflectors  than  that  which  comes  from  a  large  surface, 
which  is,  in  effect,  the  same  thing  as  coming  from  a  great 
many  different  points  at  a  greater  or  less  distance  from 
each  other.  In  former  times  a  compound  Argand  burner 
was  generally  employed,  and  is  still  in  very  extensive  use. 
This  kind  of  burner  consists  of  several  concentric  wicks — 
that  is,  circular  wicks  one  within  another — the  outer  one 
being  three  or  four  inches  in  diameter.  Such  a  light,  of 
course,  consists  of  quite  a  large  flame,  and  is  not  so  easy  to 
be  controlled  by  reflectors  or  by  lenses  as  the  same  amount 
of  light  from  a  single  point  would  be ;  so  that  when,  at 
length,  the  means  of  producing  very  bright  lights  from  a 


176  THE  CORRELATION  OF  FORCE. 

single  radiant  point — or  at  least  from  a  surface  of  very 
limited  extent,  such  as  the  oxyhydrogen  and  the  electric 
light — were  discovered,  it  was  at  once  seen  that  some  very 
great  advantages  would  result  from  employing  these  meth« 
ods  in  light-houses. 

But  there  are  other  things  to  be  considered  besides  the 
concentration  and  brilliancy  of  the  light  employed  for  this 
purpose.  The  facility  of  managing  it,  and  the  absolute  cer- 
tainty of  it — that  is,  its  entire  exemption  from  all  danger 
of  getting  out  of  order,  with  the  care  and  attention  that 
a  guardian  of  average  skill  and  fidelity  can  be  relied  upon 
to  bestow — are  points  of  essential  importance.  It  is  not 
enough,  therefore,  that  a  scientific  operator  in  his  labora- 
tory can  produce,  by  ingenious  contrivances,  a  very  pow- 
erful and  concentrated  light,  and  one  which  he  can  perfect- 
ly manage  and  control  in  a  lecture-room  for  the  amusement 
and  instruction  of  an  audience.  Before  his  plan  can  be 
adopted  by  a  Light-house  Board  they  must  know  how  his 
light  is  produced,  what  would  be  the  cost  of  it  on  a  great 
scale,  and  whether  the  apparatus  necessary  for  producing 
it  can  be  managed  safely  and  certainly  by  unscientific, 
though  careful  and  faithful  men ;  and,  more  than  all, 
whether  there  is  any  possibility  that,  even  so  seldom  as 
once  in  five  or  ten  years,  the  process  of  producing  it  might 
fail,  through  some  unexpected  derangement  of  the  appara- 
tus or  other  accident,  so  as  to  leave  the  mariners  who  might 
'be  on  the  lookout  for  it  without  its  warning  of  their  ap- 
proach to  the  land  for  several  hours,  and  perhaps  for  a 
whole  night.  There  are,  at  the  present  time,  about  six 
hundred  lights  on  the  English  coasts,  and  if  the  system 
was  such  that  a  light  was  liable  to  be  out  of  order  even 
once  in  ten  years,  that  would  make  the  number  sixty  upon 
the  average  that  would  fail  during  some  night  of  every 
year.  Such  an  uncertainty  as  this  w7ould  greatly  impair 


CAUTIONS    NECESSARY. 


177 


the  confidence  of  the  mariners,  and  vastly  increase  the 
perils  of  navigation. 

The  public  authorities  are,  therefore,  very  cautious  about 
introducing  new  modes  of  producing  light,  especially  such 
as  depend  upon  any  chemical  process.  Of  course  the  oxy- 
hydrogen  light  requires  the  preparation  of  both  oxygen 
and  hydrogen,  and  the  electric  light  that  of  a  current  of 
electricity.  The  former  renders  necessary  a  chemical  pro- 
cess involving  the  maintertance  of  a  somewhat  complicated 
apparatus,  and  a  certain  degree  of  scientific  supervision  in 
the  management  of  it.  These  constitute  insuperable  ob- 
jections to  it  in  respect  to  a  vast  majority  of  the  situations 
in  which  lights  are  required. 

These  situations  are  sometimes  quite  isolated,  light- 
houses being  not  unfrequently  built  on  rocks  at  some  di» 


178  THE  CORRELATION  OF  FORCE. 

tance  from  the  shore,  where,  in  heavy  weather,  the  sea 
breaks  over  them  with  so  much  force  that  sometimes  for 
days,  and  even  weeks,  the  keepers  are  cut  off  from  all  com- 
munication with  the  laud.  In  such  cases  it  is  very  plain 
that  the  modes  of  producing  and  managing  the  light  must 
be  of  a  very  simple  character.  The  apparatus  must  be 
very  little  subject  to  accidents  or  derangement,  and  only 
to  such  as  can  be  easily  remedied,  when  they  do  occur,  by 
persons  of  ordinary  skill. 

The  production  of  a  current  of  electricity  for  the  electric 
light  was  at  first,  and  for  a  long  time,  effected  by  a  chem- 
ical process  involving  a  considerable  degree  of  scientific 
knowledge  and  skill  in  those  directing  it,  if  not  in  its  ordi- 
nary and  successful  working,  at  least  in  the  emergencies 
which  in  all  such  operations  will  sometimes  occur. 

Within  a  somewhat  recent  period,  however,  a  method 
has  been  devised  of  developing  the  requisite  current  of 
electricity  by  means  of  mechanical  force  through  the  me- 
dium of  magnetism.  It  is  found  that  changes  in  the  mag- 
netic condition  of  an  iron  bar,  for  example,  induce  electric- 
al movements  in  any  conductors  placed  at  right  angles 
near  it.  If  an  iron  bar  is  wound  round  with  a  wire  in  a 
certain  way,  and  the  magnetic  state  of  the  bar,  while  thus 
wound,  is  made  to  change — which  may  be  easily  done  by 
alternately  bringing  it  near  and  drawing  it  away  from  a 
permanent  steel  magnet  —  a  succession  of  electrical  im- 
pulses are  induced  in  the  wire.  By  combining  many  of 
these  wound  bars — or  bobbins,  as  they  are  sometimes  called 
- — in  one  machine,  and  causing  a  number  of  permanent  mag- 
nets to  revolve  in  face  of  them  in  rapid  succession,  and  then 
combining  the  electrical  impulses  that  are  induced  in  a 
proper  manner,  the  effects,  substantially,  of  a  continued 
flow  are  secured,  so  far,  at  least,  as  is  essential  for  produc- 
ing the  electric  light. 


MAGNETO-ELECTRIC    MACHINE.  181 

The  general  principle  of  this  instrument  will  be  seen  il- 
lustrated in  the  engraving.  It  is  called  a  magneto-electric 
machine,  as  it  is  one  for  developing  electricity  by  means  of 
magnetism.  Apparatus  which  operates  on  the  converse 
principle — that  is,  of  developing  magnetism  from  electrici- 
ty, is  called  electro-magnetic. 

The  bobbins — that  is,  the  bars  of  soft  iron  wound  with 
conducting  wires — are  in  the  centre,  in  a  great  measure 
out  of  view,  being  arranged  around  the  axis  of  the  ma- 
chine. This  axis  may  be  rapidly  revolved  by  means  of 
the  band  and  pulleys  seen  above,  which  bring  in  mechan- 
ical force — provided  either  by  steam  or  horse  power — from 
the  adjoining  room.  The  ends  of  the  bars  within  the  bob- 
bins pass,  as  they  revolve,  across  the  face  of  the  poles  of 
the  powerful  horseshoe  magnets  which  are  seen  arranged 
in  tiers  on  the  outside.  There  are  eight  ranges  of  these 
magnets,  with  seven  in  a  range.  The  magnetic  condition 
of  the  iron  bars  is  changed,  of  course,  with  amazing  rapid- 
ity when  the  axis  is  made  to  revolve  at  considerable  speed. 
At  each  change  an  impulse  of  electrical  force  is  imparted 
to  the  wires  around  the  bobbins.  These  impulses  are  com- 
bined by  means  of  the  proper  connections,  and  thus  become 
practically  a  continued  stream,  which  passes  to  and  is  re- 
turned from  the  carbon  points  in  the  instrument  on  the 
left.  The  small  square  box  above  the  pedestal  contains 
the  apparatus  for  regulating  the  distance  of  the  carbon 
points  from  each  other,  this  being  necessary  to  secure 
steadiness  and  uniformity  in  the  light. 

The  whole  process,  as  exemplified  in  this  apparatus,  fur- 
nishes a  very  remarkable  and  very  comprehensive  illustra- 
tion of  a  principle  which  is  one  of  the  greatest,  if  not  the 
very  greatest  scientific  discovery  of  modern  times,  namely, 
that  of  the  correlation  of  force.  The  principle  is,  that  all 
the  great  forces  of  nature — mechanical  motion,  electricity, 


182  THE  CORRELATION  OP  FORCE. 

magnetism,  heat,  and  light,  are  modes  of  action  fundament- 
ally the  same,  in  this  sense,  namely,  that  they  are  equiva- 
lent to  each  other  in  certain  definite  proportions,  and  are 
continually  interchanging  one  into  the  other,  in  these  pre- 
cise proportions,  in  many  of  the  operations  of  nature  around 
us,  and  in  ai'tificial  processes  instituted  by  man. 

In  the  case  of  this  magneto-electric  machine  we  have  an 
example  of  all  these  forces  transforming  themselves  one 
into  another,  and  that  again  into  a  third,  till  in  this  single 
process,  taking  it  from  the  beginning  to  the  end,  we  run 
through  the  whole  round. 

To  bring  in  the  whole  of  the  circuit,  however,  we  must 
begin  with  the  sun ;  the  heat,  or  other  radiation  comprised 
in  his  beams,  is  transformed  in  the  leaves  of  the  growing 
plant  into  a  latent  force,  or  "  potential  energy,"  as  it  is  some- 
times termed,  which  is  stored  in  the  grain  or  hay  eaten  by 
the  horse,  if  we  suppose  the  machine  to  be  driven  by  horse 
power,  or  in  the  wood  which  formed  the  coal,  if  we  sup- 
pose it  carried  by  steam.  This  energy  comes  out  into  ac- 
tion in  the  animal,  developing  itself  as  muscular  force.  In 
moving  the  limbs  of  the  animal  and  turning  the  machin- 
ery, the  muscular  force  becomes  converted  into  mechanical 
motion,  which  is  conveyed  by  the  pulleys  and  bands  to  the 
magneto-electric  axis.  There  this  force  is  expended  in 
overcoming  magnetic  resistance,  and  is  transformed  by 
this  action  into  magnetic  force  in  the  soft  iron  bars,  and, 
as  it  disappears  in  this  form,  it  reappears  as  electric  force 
in  the  environing  wires.  From  these  it  passes  by  the  con- 
ducting wires  to  the  charcoal  points  where  it  is  first  trans- 
formed into  heat  to  make  incandescent  the  solid  particles 
of  carbon,  and  is  then  transformed  into  light,  to  radiate 
over  the  surface  of  the  sea.  The  whole  process  constitutes 
a  very  complete  and  striking  example  of  the  correlation,  or 
reciprocal  relation  of  the  various  forms  of  force. 


PARABOLIC    REFLECTOR. 


183 


"  CHAPTER  XXI. 


THE  most  important  object  to  be  sought,  after  determin- 
ing the  most  satisfactory  method  of  creating  a  bright  light 
for  light-house  purposes,  is,  as  has  already  been  said,  to  de- 
vise the  best  means  of  utilizing  it  when  created  by  concen- 
trating it  in  the  direction  where  it  is  required. 

In  former  times  this  was  done  by  reflectors  alone,  the 
light  being  placed  in  the  focus  of  what  is  called  a  parabolic 
reflector,  which  is  a  reflector  of  such  a  form  that  it  reflects 
the  light  radiating  from  the  focus  within  in  a  beam  of  par- 
allel rays  issuing  through  the  opening  in  front. 


PARABOLIC   REFLECTOR. 


This  is  shown  clearly  in  the  engraving,  the  black  lines 
representing  the  natural  course  of  the  rays,  and  the  dotted 
ones  the  lines  into  which  they  are  turned  by  the  reflector 
and  formed  into  a  parallel  beam. 

We  see  this  arrangement  in  operation  in  the  front  of  the 


184  FKESNEL. 

railway  locomotive  at  night,  making  the  light  far  more 
effectual  on  the  track  ahead  than  if  a  simple  lamp  without 
a  reflector,  however  bright  it  may  be,  were  vised. 

For  we  must  remember  that  light,  in  radiating  from  the 
luminous  point  through  the  atmosphere,  loses  brilliancy  as 
the  distance  increases,  from  two  causes;  first,  from  the 
spreading  or  diffusion  of  it,  on  account  of  the  divergence 
of  the  rays  from  each  other  as  they  recede,  which  causes 
the  intensity  of  it  to  diminish,  as  we  saw  in  a  former  chap- 
ter, as  the  squares  of  the  distances;  and,  secondly,  on  ac- 
count of  the  interception  of  the  light  by  solid  or  liquid  par- 
ticles always  floating  in  the  atmosphere,  which,  though  in- 
dividually invisible  to  us,  absorb  in  the  aggregate  a  great 
deal  of  light,  especially  when  the  distance  through  which 
the  ray  has  come  is  great,  so  that  it  has  had  to  encounter 
a  great  number  of  them. 

Now  there  is  no  means  of  preventing  the  loss  of  light 
from  this  latter  source,  namely,  the  absorption  of  it  by  sub- 
stances floating  in  the  air  and  thus  diminishing  the  trans- 
parency of  it.  All  that  can  be  done  is  to  increase  the 
quantity  of  light  sent  forward,  so  that  the  distance  may  be 
greater  that  will  be  required  to  absorb  it  all.  But  the 
former — that  is,  the  diminution  by  divergence — may  be  in 
a  great  measure  controlled  by  means  of  lenses  or  reflectors 
so  arranged  as  to  collect  the  light  from  all  sides,  and  send 
it  forward  over  the  sea  in  rays  diverging  but  little  later- 
ally, and  lying,  horizontally,  very  nearly  in  the  same  plane. 
It  is  evident  that  they  must  not  be  in  precisely  the  same 
plane,  for  the  surface  of  the  sea  is  not  itself  plane,  but 
slightly  convex,  on  account  of  the  rotundity  of  the  earth. 
The  rays,  of  course,  can  not  be  made  to  curve  in  their 
course  to  accommodate  themselves  to  this  rotundity,  and 
so  it  is  necessary  that  they  should  diverge  a  little  upward 
and  downward  in  order  that  they  may  shine  upon  both 


FUNCTION    OF   THE    REFLECTOR.  185 

near  and  distant  vessels ;  and  they  must  also  diverge  to  a 
considerable  extent  from  side  to  side,  so  that  they  may 
reach  every  ship,  whatever  may  be  the  direction  from  which 
she  approaches  the  land. 

Reflectors  are  necessary  for  all  that  portion  of  the  ra- 
diance which  would  naturally  proceed  toward  the  land,  for 
it  is  only  by  reflection  that  light  can  be  turned  back  direct- 
ly from  its  course. 

For  a  long  time  reflectors  alone  were  used  for  the  man- 
agement of  the  light  in  these  cases.  They  only  served  the 
purpose,  of  course,  of  intercepting  and  turning  forward  that 
portion  of  the  radiation  which  was  emitted  on  the  side  op- 
posite to  that  on  which  it  was  required.  The  light  which 
naturally  went  forward  was  left  to  pursue  its  own  course 
without  modification.  It  could  only  be  modified  by  the 
use  of  lenses,  and  the  difficulty  of  constructing  lenses  of  a 
sufficient  size  for  the  purpose  was  for  a  long  time  insur- 
mountable. 

Contrivances  for  reflecting  the  light  were,  however,  very 
numerous,  and  some  of  them  were  very  ingenious  and  very 
complicated. 

The  engraving  on  the  following  page  represents  a  sys- 
tem of  reflectors  devised  to  produce  Avhat  is  called  a  flash- 
ing light;  for,  in  the  case  of  beacon-lights  that  are  not 
many  miles  apart,  the  luminous  effects  must  be  made  to 
differ  in  some  way,  in  order  to  prevent  their  being  mis- 
taken for  each  other.  There  are  a  great  many  ways  of 
making  these  variations.  The  light  may  be  colored  by  be- 
ing caused  to  pass  through  red,  green,  or  blue  glass.  It 
may  be  revolving  or  intermittent,  or  may  be  sent  forth  in 
flashes.  In  the  engraving  the  several  reflectors  have  each 
its  own  lamp,  and  they  are  arranged  in  sets  of  three  (A,  B, 
C)  upon  a  vertical  axis,  which  is  made  to  revolve  by  ap- 
proximate machinery,  as  indicated  by  the  pulley  on  the 


186 


FRESNEL. 


FLASHING   I.IGIIT   BY   REFLECTOI 


left.  The  cord  D,  descending  from  the  pulley,  passes  over 
another  pulley  not  seen,  and  has  a  weight  like  a  clock- 
weight  attached  to  it.  The  clock-work  by  which  the  de- 
scent of  the  weight  is  regulated  and  the  force  communica- 
ted to  the  system  of  reflectors  is  inclosed  in  the  box  E. 
The  little  truck  wheels  on  which  the  system  revolves  are 
seen  beneath,  on  the  platform  formed  upon  the  top  of  the 
stand,  where  they  roll  in  a  groove  formed  for  the  purpose. 
The  machinery  is  arranged  so  that  the  lamps  revolve 


IMPROVEMENTS    UPON   FKESNEI/S    SYSTEM.  187 

very  slowly,  and  all  that  is  required  to  secui-e  the  steady 
continuance  of  motion  is  that  the  clock-work  should  be 
wound  up  every  day  when  the  lamps  are  trimmed,  and  set 
a-going  when  the  lamps  are  lighted  at  night.  Of  course, 
as  each  set  of  three  lamps  comes  to  the  front,  their  com- 
bined light  sends  a  flash  far  over  the  sea. 

A  mode  of  constructing  lenses  of  a  size  sufficient  to  be 
used  in  light-houses  was  at  last  devised  by  a  French  phil- 
osophical engineer  named  Fresnel;  and  so  great  was  the 
success  of  his  system,  that  it  came  soon  to  be  almost  uni- 
versally introduced,  and  has  connected  his  name  indissolu- 
bly  with  the  light-house  system  all  over  the  world.  Very 
great  improvements  have  been  made  in  his  system  by  oth- 
er inventors,  but  they  do  not  displace  his  name  as  the  orig- 
inator of  the  idea  out  of  which  they  have  all  proceeded. 
.  It  is  only  an  idea  of  the  general  principle  of  Fresnel's  in- 
vention that  can  be  given  in  a  chapter  like  this.  It  con- 
sists essentially  in  "building  up,"  as  it  were,  a  lens  for  the 
concentration  of  the  rays,  by  forming  it  in  separate  por- 
tions, each  portion  except  the  central  one,  and  sometimes 
even  that,  being  in  the  form  of  a  ring,  the  surfaces  of  all 
the  portions  being  so  arranged  as  to  produce  the  same  ef- 
fect in  refracting  the  rays  as  if  the  lens  was  made  in  one 
solid  mass. 

To  'understand  this  clearly,  we  must  consider  that  the 
function  which  it  is  required  of  the  lens  to  perform  is  to 
draw  in  the  rays  somewhat  from  their  natural  divergence, 
since  in  issuing  from  the  source  they  would,  if  left  to  them- 
selves, diverge  too  widely.  Now  it  is  the  property  of  a 
convex  lens  to  produce  this  effect,  as  we  see  exemplified 
in  the  case  of  the  sun-glass,  so  called,  which  is  often  used 
as  a  toy  to  concentrate  the  light  and  heat  of  the  sun. 

Now,  to  make  a  lens  of  this  form,  and  of  the  size  which 
would  be  necessary  for  a  light-house,  would  require  a  very 


188 


CONVERGENCE  OF  EAYS. 


great  thickness  of  glass  in  the  central  parts,  all  of  which 
thickness  would  be  useless  in 
itself,  since  the  light,  in  pass- 
ing through,  is  changed  in  its 
direction  only  at  the  surfaces 
where  it  enters  and  where  it 
emerges.  It  undergoes  no 
change  of  direction  while  it  is 
passing  through  the  substance 
of  the  glass  within.  In  other 
words,  the  whole  effect  of 
bending,  or,  as  it  is  scientific- 
ally termed,  of  refracting  the  rays,  depends  upon  the  angle 
of  inclination  in  respect  to  the  surface  of  the  glass  at  which 
the  ray  enters  and  leaves  it.  Fresnel's  idea  was,  therefore, 
to  dispense,  as  far  as  possible,  with  the  interior  substance 
of  the  glass,  by  dividing  the  lens  into  portions,  and  making 
the  several  portions  thin,  while  he  still  preserved  in  all  the 
same  inclinations  of  the  surfaces  in  relation  to  the  entering 
and  departing  ray. 

You  will  see  how  this  object  is  effected  by  the  engrav- 
ing, which  shows  pretty  clearly  the  nature  of  Fresnel's 
contrivance,  and  the  manner  in  which  it  operates  to  pre- 
serve all  the  refracting  power  of  a  convex  lens  by  retain- 
ing the  several  portions  of  the  surface  in  the  right  position 
in  respect  to  the  entering  and  departing  ray,  while  yet  the 
thickness  of  the  glass  is  kept  within  reasonable  limits. 


DIFFERENT   FORMS.  189 

The  rays  of  light  from  the  charcoal  points  to  the  left  are 
brought  into  a  parallel  beam  as  they  leave  the  lens  on  the 
right. 

In  this  case,  the  rays,  diverging  first  from  a  point,  become 
parallel,  which  is  the  reverse  of  the  case  of  the  sun-glass, 
in  which  parallel  rays — that  is,  rays  sensibly  parallel  on  ac- 
count of  the  sun's  great  distance — are  made  to  converge  to 
a  point.  All  refraction  is  in  this  way  reciprocal.  Rays 
subjected  to  refraction  will  always  follow  the  same  track 
in  this  sense,  namely,  that  if  they  enter  the  glass — on  the 
right  side,  for  example — coming  in  a  certain  direction,  and 
go  out  on  the  other  side  in  a  certain  different  direction, 
then,  if  the  motion  is  reversed,  and  the  rays  of  another 
beam  come  in  on  the  left  as  the  former  went  out,  they  will, 
on  refraction,  in  the  same  lens,  go  out  on  the  right  precise- 
ly as  the  former  came  in. 

It  is  plain  that  a  lens  modified  on  Fresnel's  system,  as 
above  described,  might  be  made  of  a  circular  form,  as 
usual,  in  which  case  several  sets  of  them  would  be  required, 
forming  diiferent  faces,  to  be  presented  toward  different 
quarters  of  the  horizon.  Or  the  lens  might  be  made  annu- 
lar, with  a  broad  convex  surface  in  the  centre,  and  narrow 
ones  in  rings  above  and  below.  This  last  arrangement  is 
shown  in  a  simple  form  in  the  engraving  on  the  following 
page,  which  represents  a  signal,  lantern  such  as  is  used  on 
board  ships. 

These  engravings  are  copies  of  those  which  Lawrence 
gave  to  John  to  put  into  his  note-book,  as  illustrating  in  a 
simple  form  the  fundamental  principle  of  Fresnel's  idea. 
John  afterward  found,  when  he  came  to  visit  light-houses 
on  the  coasts  of  France  and  England,  that,  in  carrying  the 
idea  into  practical  effect,  a  great  number  and  variety  of 
most  elaborate  and  complicated  arrangements  were  made. 
When  he  Avent  inside  of  some  of  the  large  lanterns  and 


190 


SIGNAL   LAKTKBN. 


looked  around  at  the  vast  number  of  angular  rings  of  glass 
— that  is,  rings  angular  in  section — and  prisms,  and  groups 
of  lenses  and  reflectors,  he  was  sometimes  utterly  bewil- 
dered with  the  intricacy  of  the  system,  and  almost  dazzled 
by  the  brilliancy  of  the  effect  produced  by  so  much  pol- 
ished glass,  even  in  the  daytime,  when  the  lamps  were  not 
lighted. 

He  found  that  not  only  lenses,  modified  as  above  de- 
scribed, were  used,  but  prisms  of  the  same  annular  form, 
placed  above  and  below  the  limits  of  the  lenses,  were  em- 
ployed to  bring  down  into  parallelism  rays  which  would 
otherwise  have  passed  out  of  range.  Ho\v  this  is  done  will 


PRISMS. 


191 


be  shown  by  the  engraving,  where  the  rays  forming  the 
centre  portion  of  the  diverging  beam  are  brought  to  paral- 
lelism by  the  lens,  and  those  that  ascend  and  descend  much 
are  partly  refracted,  but  mainly  reflected,  by  the  prisms 
placed  in  proper  positions  for  this  purpose,  as  shown  in 
section  in  the  engraving. 


EFFECT   OF   TIIE   I'RIGMS. 


You  will  observe  from  the  engraving  that  the  light  is 
reflected  from  the  under  side  of  the  upper  surface  of  the 
prism,  where  we  should  naturally  think  it  would  emerge. 
It  is  very  remarkable  that  it  should  be  thus  reflected  back 
through  the  interior  of  the  glass  again,  instead  of  going 
out  into  the  air ;  but  such  is  the  fact.  We  can  see  a  strik- 
ing example  of  reflection  of  this  kind  by  means  of  a  tum- 
bler of  water.  Fill  a  tumbler  nearly  full,  then  hold  it  up 
carefully  above  your  head,  and  look  up  at  the  under  surface 
of  the  water ;  you  will  find  that  you  can  not  see  through 
it  to  what  is  above.  If,  while  you  hold  the  tumbler  in  one 
hand,  you  hold  the  finger  of  the  other  hand  above  the  top 


192  FRESNEL. 

of  it,  you  can  not  see  it  by  looking  up  through  the  upper 
surface;  but  if  you  bring  the  finger  down  below  the  level 
of  the  water,  and  on  the  farther  side  of  it,  then,  with  a  lit- 
tle care  in  placing  it  right,  you  will  see  it  reflected  in  the 
upper  surface  seen  from  below — that  is,  from  the  under  side 
of  the  upper  surface. 

These  explanations  will  give  the  reader  a  general  idea 
of  the  fundamental  principles  of  Fresnel's  invention  for 
managing  the  light  in  light-houses,  and  will  enable  any 
one,  when  he  visits  a  light-house  constructed  on  these  prin- 
ciples, to  understand  what  he  sees,  when,  without  this  pre- 
liminary knowledge,  the  complicated  combination  of  rings 
of  glass,  and  mirrors,  and  prisms  would  form  for  him  only 
an  intricate  and  bewildering  maze. 

Indeed,  the  number  and  the  variety  of  the  modes  in 
which  these  general  principles  are  applied,  and  the  vast 
extension  which  the  system  has  received  since  Fresnel  first 
inti'oduced  it,  and  wrhich  is  necessary  to  produce  the  great 
variety  of  luminous  effects  required  for  distinguishing  the 
different  lights  from  each  other,  are  such  that  it  is  the 
work  of  a  lifetime  to  understand  the  whole  subject  in  all 
its  details. 

Fresnel  was  a  highly-educated  man  and  a  profound  math- 
ematician, and  he  made  his  discoveries,  not  by  any  lucky 
accident,  but  by  the  most  careful  and  thorough  study  of 
the  philosophy  of  optics.  He  was  educated  as  an  engineer 
in  the  military  schools  of  France,  and  was  subsequently 
appointed  to  important  posts  under  the  French  govern- 
ment— first  in  respect  to  bridges  and  roads,  and  afterward 
in  relation  to  the  establishment  and  management  of  light- 
houses on  the  coasts.  It  was  from  the  profound  investi- 
gations that  he  made  in  connection  with  his  official  duties 
that  his  discoveries  and  inventions  resulted. 

And  yet,  notwithstanding  the  great  eminence  as  a  math- 


PRACTICAL   REFLECTIONS.  193 

ematician  and  philosopher  to  which  he  attained,  he  was, 
when  a  boy  at  school,  considered  quite  a  dull  scholar,  on 
account  of  his  apparent  incapacity  for  learning  and  recit- 
ing lessons  by  rote,  as  was  then,  and  still  is,  much  practiced 
in  schools.  Yet  he  was,  even  at  that  early  age,  so  much 
interested  in  the  study  of  philosophical  principles,  that  he 
had  the  name  and  reputation  of  a  genius  among  his  play- 
mates, on  account  of  his  success  in  investigating  the  action 
and  improving  the  forms  of  their  toys  and  playthings,  such 
as  their  tops,  kites,  and  little  cannon. 

If  I  were  writing  a  moral  discourse  in  the  form  of  a  ser- 
mon instead  of  a  scientific  treatise,  I  might  very  properly 
close  this  chapter  with  two  practical  reflections. 

First,  that  a  boy,  because  he  thinks  himself  smart  in 
learning  and  reciting  lessons  at  school,  should  not,  on  that 
account,  become  conceited  and  vain,  and  imagine  that  he  is 
certainly  going  to  become  a  great  man  when  he  grows  up. 
Intellectual  success  and  distinction  in  future  life  depends 
upon  something  very  diiferent  from  mere  readiness  in  com- 
mitting to  memory,  and  fluency  in  repeating,  mere  words. 

And,  secondly,  if  any  boy  who  is  patient,  faithful,  and 
thoughtful  in  his  endeavors  to  understand  what  he  is 
taught,  but  finds  that  he  is  not  so  quick  and  ready  in 
learning  and  reciting  the  lessons  as  others  in  his  class,  he 
has  no  occasion  to  be  discouraged  about  himself.  He  may 
have  within  him  all  the  essentials  of  eminent  success  in  the 
acquisition  of  knowledge,  which  will  develop  themselves  in 
due  time. 

I 


194 


CHAPTER  XXII 


NOTHIXG  can  be  in  appearance  more  simple  and  uncom- 
pounded,  or,  as  it  is  scientifically  expressed,  more  appar- 
ently homogeneous,  than  pure  white  light.  It  was  the 
celebrated  Sir  Isaac  Newton  who  first  called  the  attention 
of  mankind  strongly  to  the  fact  that  a  beam  of  such  light 
can  be  separated  into  many  component  parts,  strikingly 
different  from  each  other  in  their  powers  and  properties  in 
relation  to  human  vision.  It  has  since  been  discovered 
that  the  radiation  from  the  sun,  which  was  formerly 
thought  to  consist  of  simple  beams  of  light  and  heat,  is 
infinitely  more  complicated  than  even  Newton  imagined, 
who  only  separated  the  beam  of  light  into  the  seven  prin- 
cipal colors. 

The  discovery  of  Newton  was  this:  "When  light  passes 
out  of  a  rare  medium  like  air  into  a  dense  one  like  glass, 
or  water,  or  ice — or  reversely,  from  any  such  substance 
into  air,  if  it  enters  or  emerges  obliquely,  the  ray  is  bent 
a  little  out  of  the  direct  line,  as  you  see  very  clearly  when 
you  hold  a  pole  or  stick  in  an  oblique  position,  with  the 
lower  end  of  it  in  the  water.  The  stick,  seen  from  above, 
appears  bent  at  the  place  where  it  enters  the  water,  on  ac- 
count of  the  rays  which  come  from  that  part  of  it  which 
lies  under  the  water  being  turned  somewhat  downward  as 
they  emerge,  and  thus  are  made  to  enter  the  eye  as  if  they 
came  from  higher  points  within  the  water  than  they  actu- 
ally do  come  from. 

For,  as  has  already  been  explained,  things  always  appear 


EFFECT   OF    REFRACTION. 


195 


POLE   8EEMLNG   TO  BE  BENT. 


to  us  in  the  place  from  which  the  rays  seem  to  come  when 
they  enter  the  eye. 

This  refracting  effect  upon  rays  passing  in  an  oblique 
direction  into  or  out  of  surfaces  of  water  or  glass  has  long 
been  known.  Indeed,  it  gives  rise  to  phenomena  which 
are  to  be  observed  all  around  us  every  day.  Newton, 
however,  in  his  investigations,  ascertained  that  the  light 
thus  bent  out  of  its  course  was,  in  some  mysterious  way, 
separated  in  the  bending  into  different  component  parts. 

The  engraving  on  the  following  page  represents  this 
phenomenon  in  its  simplest  form.  A  beam  of  light  from 
the  sun  enters  through  an  orifice  (a)  made  in  the  shutter 
into  a  darkened  room ;  for,  though  this  is  not  essential,  the 
effect  is  far  more  decided  when  all  other  light  except  the 
beam  to  be  experimented  upon  is  excluded.  The  ray,  in 
entering  from  the  air  into  the  prism  (shown  in  section  in 
the  engraving)  at  #,  is  refracted  upward,  and  slightly  sep- 


196  COLOK. 

arated  into  differently-colored  parts.  In  coming  out  of 
the  prism  on  the  other  side  at  o,  it  is 
refracted  more,  and  the  component  col- 
ored portions  are  still  more  widely  sep- 
arated. They  go  on  diverging  as  they 
recede,  until,  when  they  at  length  fall 
upon  the  wall  at  d,  or  upon  a  screen 
placed  there  to  receive  them,  the  rays 
.KHKAOTIONOKBOLAKBAY.  which  entered  at  a  as  a  small  beam  of 
white  light  form  an  elongated  band  of  the  most  brilliant 
and  beautiful  colors. 

This  band  is  called  a  spectrum.  In  this  case  it  is  the 
spectrum  of  solar  light,  or  the  solar  spectrum,  that  is  pro- 
duced. The  spectrum  of  any  other  light  may  be  produced 
by  similar  means. 

In  case,  however,  the  light  to  be  employed  radiates  from 
a  point  that  is  near,  so  that  the  rays  are  divergent  instead 
of  being  parallel  like  the  sun's  rays,  there  is  an  advantage 
in  passing  them  first  through  a  convex  lens  to  bring  them 
to  parallelism  before  they  enter  the  prism. 

The  number  of  colors  which  were  developed  in  the  solar 
spectrum  by  Newton's  experiments  were  seven,  and  were 
called  for  a  long  time  the  seven  primitive  colors.  It  was 
found  that,  by  mingling  these  hues  again  by  any  suitable 
means,  the  white  light  from  which  they  originated  was  re- 
produced. Thus  the  beam  of  white  light,  it  was  found, 
could  be  separated  by  refraction  into  rays  of  seven  differ- 
ent colors,  and  these,  by  being  combined  again,  would  re- 
produce the  original  beam  of  white  light. 

There  are  many  ways  by  which  this  reproduction  can 
be  effected.  One  is  by  gathering  the  rays  together  again 
from  the  spectrum  by  means  of  a  lens,  or  of  another  prism, 
in  a  reversed  position.  Another  is  by  mixing  paints  of  the 
seven  hues  together  upon  a  painter's  pallet.  A  third  mode 


THE    PHILOSOPHICAL   TOP.  197 

is  by  causing  a  disk  with  different  sec- 
tions  of  it,  or,  rather,  different  sectors, 
colored  differently,  to  revolve  rapidly, 
by  means  of  a  top,  for  example,  so  as 
to  mingle  and  blend  the  colors  in  their 
impression  upon  the  retina  of  the  eye. 
Of  course,  in  both  these  last  experiments,  in  order  to  se- 
cure complete  success,  it  is  necessary  that  the  colors  to  be 
combined  should  be  of  the  same  hues  and  in  the  same  pro- 
portions as  those  developed  in  the  spectrum  by  the  decom- 
position of  the  pure  white  light  of  the  solar  beam. 

A  top  of  suitable  form,  as  affording  a  ready  means  of 
producing  a  rapid  rotation,  answers  very  well  for  making 
experiments  in  the  blending  of  colors.  Indeed,  with  a  lit- 
tle ingenuity,  a  top  may  be  contrived  so  that  different  disks 
may  be  fitted  to  it,  and  thus  a  variety  of  experiments  may 
be  made.  The  method  which  Newton  adopted,  however, 
was  somewhat  more  systematic  than  this.  He  constructed 
a  little  machine  to  which  his  disks  could  be  fitted,  and  thus 
made  to  revolve  very  rapidly  by  means  of  a  multiplying 
wheel — that  is,  a  large  wheel  turning  a  small  one  by. a 
band. 

The  figure  on  the  left,  on  the  next  page,  represents  the 
disk  divided  into  sectors  by  lines  drawn  from  the  centre 
to  the  circumference,  the  several  divisions  being  painted  in 
the  colors  which  it  is  desired  to  blend.  "When  this  disk  is 
put  upon  the  little  axle  made  to  carry  it,  in  the  machine, 
and  set  in  rapid  revolution,  if  the  colors  are  of  the  right 
hue  and  properly  proportioned,  they  all  disappear,  and  the 
whole  surface  becomes  apparently  white,  as  shown  in  the 
central  figure  of  the  following  engraving. 

In  process  of  time,  as  the  solar  spectrum  was  more  close- 
ly examined,  and  as  the  instruments  for  producing  it  were 
made  more  perfect,  and  the  arrangements  for  performing 


198 


the  experiment  were  improved,  the  spectrum  became  more 
and  more  enlarged,  and  the  colors  more  separated,  until  at 
length  certain  mysterious  dark  lines  began  to  appear,  pass- 
ing across  it  in  various  places  like  black  bars  or  bands 
upon  a  colored  ground.  These  bands  were  found  to  be 
fixed  and  permanent — that  is,  they  were  always  the  same, 
and  appeared  in  precisely  the  same  positions  in  the  spec- 
trum, by  whomsoever  and  wheresoever  produced.  A  cele- 
brated optician  of  Germany,  named  Fraunhofer,  first  called 
the  general  attention  of  the  scientific  world  to  this  phe- 
nomenon, and  after  a  time  he  published  a  colored  map  of 
the  spectrum  with  these  lines  represented  upon  it.  They 
were  thenceforth  known  in  the  scientific  world  as  Friiun- 


THE    SPECTRUM.  199 

hofer's  lines,  and  they  excited  great  interest,  though  the 
cause  and  the  significance  of  them  remained  for  a  long  time 
an  unfathomable  mystery. 

Fraunhofer  gave  names  to  the  principal  lines  that  he  ob- 
served as  soon  as  he  found  that  they  were  con- 
stant and  unchangeable  in  the  solar  spectrum, 
designating  each  by  a  letter  of  the  alphabet. 
The  names  and  positions  of  some  of  the  princi- 
pal ones — those  that  were  first  discovered — are 
1 1  given  in  the  engraving. 

The  white  band  on  the  black  ground  repre- 
sents the  spectrum  itself,  and  the  lettered  cross- 
bars Fraunhofer's  lines.  It  is  only  a  very  few  of 
the  principal  ones  that  are  thus  named.  Fraun- 
hofer himself  discovered  and  mapped  many  hun- 
dred of  them,  and  the  number  has  since  been  ex- 
tended to  several  thousand. 

It  is  now  found  that  these  lines  depend  upon  the  nature 
and  the  chemical  composition  of  the  incandescent  sub- 
stance from  which  the  light  proceeds.  They  are  always 
the  same  for  the  light  which  comes  from  the  sun,  but  dif- 
ferent for  the  different  kinds  of  artificial  light ;  always  the 
same,  however,  for  the  same  kind  of  light.  There  are  cer- 
tain lines,  it  is  found,  that  are  characteristic  of  certain  sub- 
stances, and  when  these  substances  exist  in  any  flame,  or 
other  incandescent  source  of  light,  the  lines  pertaining  to 
them  ai-e  sure  to  appear.  Thus  these  lines  in  any  spec- 
trum constitute  a  language  by  which  those  who  have 
learned  it  can  determine  with  great  certainty  what  sub- 
stances exist  in  any  flame  the  spectrum  of  which  they  have 
the  opportunity  to  examine. 

To  examine  the  spectra  of  flames  in  this  way  requires  a 
somewhat  complicated  and  delicate,  and  very  exactly  made 
instrument.  This  instrument  is  called  the  spectroscope. 


200 


COLOE. 


The  form  and  general  appearance  of  it  is  shown  in  the  an- 
nexed engraving. 


THE   SPECTROSCOPE. 


It  is  only  a  very  general  idea  of  the  character  and  con- 
struction of  the  instrument  that  it  is  necessary  to  commu- 
nicate here.  It  consists  substantially  of  a  prism  in  the 
centre,  supported  upon  a  stand,  and  three  branches  in  the 
form  of  telescopes  directed  toward  it.  The  one  on  the 
right  is  for  the  light  which  is  to  be  examined,  Avhich  passes 
through  the  tube,  and  is  prepared  by  the  lenses  contained 
in  it  for  the  prism  in  which  the  spectrum  is  formed.  The 
spectrum  is  viewed  by  means  of  the  telescope,  which  con- 


THKEE    DIFFERENT    EMANATIONS.  201 

stitutes  the  branch  on  the  left.  The  third  branch  contains 
a  micrometer  scale,  so  called,  an  image  of  which  is  project- 
ed upon  the  spectrum,  and  enables  the  observer  to  deter- 
mine the  precise  position  of  any  bands  which  may  be 
brought  to  view.  By  observations  made  with  this  instru- 
ment— spectral  analysis,  as  it  is  called — the  radiation  which 
comes  from  the  sun,  as  well  as  that  from  many  other  lu- 
minous sources,  is  found  to  be  of  an  exceedingly  complex 
character.  The  portion  of  it  which  constitutes  light — that 
is,  which  has  the  power  of  producing  in  our  minds  the  sen- 
sation of  vision,  can  be  separated  into  a  great  number  of 
distinct  parts,  each  of  which  produces  a  different  sensation 
in  our  minds  in  respect  of  color;  and,  besides  the  light, 
there  are  rays  of  heat,  and  a  third  kind  still,  called  chem- 
ical rays,  a  considerable  portion  of  both  of  which  can  be 
entirely  separated  from  the  rays  of  light  in  the  spectrum. 
Fifty  years  ago,  a  commonly  received  theory  was  that  all 
this  radiation  consists  of  streams  of  solid  particles,  which 
were  both  inconceivably  minute,  and  were  projected  from 
their  source  with  inconceivable  velocity.  The  prevailing 
theory  at  the  present  day  is  that  they  consist  of  different 
modes  of  vibration  or  undulation,  in  an  extremely  rare  and 
tenuous  ether,  wrhich  is  supposed  to  pervade  all  space,  and 
even  to  fill  the  interstices  of  solid  bodies.  The  longer  and 
more  slow  of  these  vibrations — if  the  words  long  and  slow 
can  be  applied  at  all  to  movements  so  amazingly  minute 
and  rapid  as  these  are  all  supposed  to  be — constitute,  it  is 
thought,  the  rays  of  heat.  Those  of  somewhat  greater  ve- 
locity and  minuteness  form  the  light,  while  the  most  minute 
and  rapid  of  all  are  the  rays  of  chemical  action.  In  other 
words,  that  the  vibrations  of  the  first  class — that  is,  those 
that  are  comparatively  slow,  have  the  power  to  affect  us 
with  the  sensation  of  heat ;  those  of  the  second  class  have 
the  property  of  producing  impressions  upon  our  organs  of 
12 


202  COLOR. 

vision ;  while  the  third,  in  some  mysterious  way,  act  upon 
the  principles  of  chemical  affinity. 

And  here  it  is  proper  to  say  that,  in  reading  works  of 
science  and  philosophy,  we  must  keep  clearly  in  mind  the 
distinction  between  the  facts  which  are  brought  to  light  by 
the  observations  and  experiments  of  scientific  men,  and  the 
theories  by  which  they  attempt  to  explain  them.  Facts, 
once  established  by  proper  evidence,  remain  uncontrovert- 
ed  from  age  to  age.  We  can  rely  with  confidence  upon 
them.  But  the  theories  are  continually  changing.  They 
are  only  suppositions  which  may  be  imagined  to  account 
for  the  facts,  and  ought  to  be  received  with  great  caution. 
There  is  direct  and  positive  proof  that  sound  is  produced 
by  vibrations  of  some  material  substance,  but  there  is  no 
such  direct  proof  in  respect  to  luminous  radiations.  It  is 
only  a  matter  of  inference  and  reasoning.  The  reasoning 
is,  that  we  can  only  conceive  of  two  modes  by  which  a 
force  can  be  transmitted  through  space,  namely,  by  the 
progressive  motion  of  material  particles,  and  by  an  imdu- 
latory  movement  of  an  intervening  medium  /  and  as  it  has 
been  abundantly  proved  that  it  is  not  and  can  not  be  the 
former,  we  may  safely  infer  that  it  must  be  the  latter.  It 
may  be  so ;  but  then,  on  the  other  hand,  it  may  be  sup- 
posed possible  that  modes  of  the  transmission  of  force  can 
exist  of  which,  having  no  experience  of  them,  we  can  have, 
in  our  present  state  of  knowledge,  no  conception.  It  is  not 
very  safe  for  minds  as  limited  in  their  attainments  and 
powers  as  ours  to  conclude  that  a  phenomenon  that  we 
witness  must  necessarily  be  accomplished  in  one  of  two 
ways  simply  from  the  fact  that  we  do  not  know  of  any 
third. 

However  this  may  be,  the  scientific  world  at  the  present 
day  are  almost  universally  convinced  that  the  phenomena 
of  light,  heat,  electricity,  and  the  like,  are  all  the  results  of 


LIGHT   A   FOBM    OP   FORCE.  203 

a  vibratory,  or,  as  one  of  them  expi-esses  it,  a  kind  of  shiv- 
ering motion  of  the  particles  of  matter.  But  perhaps  they 
are  not  more  generally  agreed  in  accepting  this  view  now 
than  they  were  in  holding  to  the  belief  that  these  several 
principles  were  so  many  distinct  material  substances  half 
a  century  ago. 

But,  whatever  the  truth  may  be  in  regard  to  the  theory, 
there  is  no  doubt  of  the  fact  that  light  is  a  form  of  foi-ce, 
which  is  transmitted  from  the  sun  or  other  luminous  cen- 
tre in  combination  or  connection  with  heat  and  with  actin- 
ism— as  the  chemical  force  is  called — in  apparently  simple 
and  homogeneous  rays,  which,  however,  in  consequence  of 
their  different  degrees  of  refrangibility,  may  be  separated 
from  each  other,  the  ray  of  light  itself  being  resolvable 
into  seven  or  more  component  portions,  which  have  re- 
spectively the  power  of  producing  in  us  sensations  of  the 
several  colors.  When  any  one  of  these  rays  enter  the  eye 
directly  from  the  prism  which  has  separated  them,  it  pro- 
duces at  once  and  directly  the  appropriate  sensation.  If 
the  spectrum  falls  upon  a  screen,  or  upon  any  surface  serv- 
ing as  a  screen,  the  several  portions  of  it  are  reflected,  and, 
entering  the  eye,  they  produce  each  its  own  proper  sensa- 
tions in  this  secondary  manner. 

The  rays  of  light,  besides  being  separable  into  their  com- 
ponent portions  in  this  way  by  refraction  in  passing  from 
one  transparent  medium  into  another,  are  also  subject  to 
a  somewhat  similar  modification — similar,  at  least,  in  some 
respects — when  falling  upon  any  opaque  substances.  Some 
such  substances  absorb  the  whole  of  the  light,  and  reflect 
none  of  it  to  the  eye ;  these  are  black  substances.  Others 
reflect  the  whole,  though  in  a  peculiar  manner ;  these  are 
white  substances.  Others  absorb  certain  portions  of  the 
separated  rays  and  reflect  others  of  them,  as  the  green 
rays,  or  the  blue  rays,  for  example— that  is,  the  rays  that 


204  COLOR. 

are  capable  respectively  of  exciting  the  green  or  the  blue 
sensation  in  our  minds  when  they  actually  enter  our  eyes. 
That  they  are  not  green  or  blue  themselves,  but  only  have 
the  power  of  exciting  these  sensations  in  us,  is  evident  from 
the  fact  that,  when  we  look  out  at  a  window  through  the 
air,  although  the  blue  rays,  as  we  call  them,  are  coming 
down  through  it  all  the  time  from  the  sky  above,  and  the 
green  ones  coming  up  through  it  from  the  grass  below,  we 
see  no  blue  or  green  unless  we  turn  our  eyes  toward  the  sky 
or  the  ground.  In  other  words,  the  r&ys  have  no  blueness 
or  greenness  in  themselves,  but  only  have  the  power  of 
producing  the  peculiar  sensations  to  which  we  give  those 
names  when  they  enter  our  eyes  and  take  effect  upon  the 
sensitive  organization  of  the  retina. 

It  is  substantially  the  same  with  transparent  substances 
of  different  colors.  Green  glnss,  for  example,  absorbs  all 
those  portions  of  the  solar  ray  which  have  not  the  power 
of  producing  in  us  the  sensation  of  green,  while  they  allow 
those  that  have  this  power  to  pass.  Accordingly,  any 
thing  that  we  see  through  such  glass  appears  tinged  with 
green. 

Thus  the  color  of  any  substance,  transparent  or  opaque, 
depends  upon  the  part  of  the  solar  ray  which  it  reflects  or 
transmits.  And  this  is  the  philosophy  of  color,  as  at  pres- 
ent understood. 


ILLUSIONS.  205 


CHAPTER  XXIII. 

FLIPPY. 

I  HAVE  some  doubt  whether  the  readers  of  this  book 
will  be  easily  convinced  of  the  truth  of  what  I  am  going 
to  show  in  this  and  the  next  chapter;  but  if  they  are  not 
convinced  of  it  at  once  now,  I  am  sure  that  they  will  be  in 
time,  as  they  grow  older  and  think  more. 

There  is  nothing  that  we  are  more  inclined  to  trust  in 
than  the  evidence  of  our  senses,  and,  of  all  the  others,  there 
is  none  that  inspires  us  with  more  confidence  than  that  of 
sight;  and  yet  there  is  no  one  of  them  all  that  is  so  falla- 
cious, or  that  produces  in  us  so  many  illusions. 

When  we  are  little  children  we  see  a  reflection  in  the 
looking-glass.  We  think  there  is  something  behind  the 
glass.  We  look,  and  find  nothing  there.  It  is  an  illusion. 

As  we  grow  older  we  know  that  there  is  nothing  behind 
the  glass,  but  we  are  apt  to  imagine  that  there  is  an  image 
or  picture  somehow  or  other  in  it.  It  is  an  illusion  just 
like  the  other.  There  is  no  image  or  picture  of  any  kind 
in  the  glass;  the  image  is  in  our  eyes,  and  nowhere  else. 

We  look  up  at  the  sky  at  night — or,  in  fact,  in  the  day- 
time when  the  sun  is  not  too  bright — and  think  we  see  a 
grand  arch  swelling  above  us.  There  is  really  no  arch 
there;  it  is  all  an  illusion. 

We  look  at  the  dark  cloud  in  the  east,  when  a  shower 
is  past  and  the  sun  shines  out  upon  the  cloud  from  the 
west,  and  think  we  see  a  rainbow  there.  Illusion !  there 
is  no  rainbow  except  in  our  eyes.  There  are  causes  in  op- 
eration in  the  cloud  that  produce  the  image  of  a  rainbow 
i:i  our  eyes,  and  that  is  all. 


206  FLIPPY. 

"We  look  around  us  upon  a  spring  or  summer  day,  and 
see,  as  we  think,  greenness  in  the  grass,  and  other  beauti- 
ful colors  in  the  flowers.  Illusion !  the  colors  are  sensa- 
tions in  our  minds. 

The  sense  of  sight  is  perhaps  the  most  fruitful  source  of 
our  illusions,  but  the  other  senses  are  in  some  cases  equally 
deceptive.  We  hold  our  hands  before  the  fire  and  experi- 
ence a  sensation  of  warmth,  and  we  imagine  that  there  is 
warmth  in  the  fire;  but  a  moment's  reflection  will  show 
us  that  warmth  is  a  sensation,  and  that  there  can  not  pos- 
sibly be  a  sensation  of  any  kind  in  fire.  It  is  sometimes 
said  that  there  is  no  heat  in  fire,  and  this  is  true  if  we  mean 
by  heat  the  sensation  of  heat  such  as  we  ourselves  experi- 
ence by  the  action  of  fire.  But  the  word  heat  has  a  double 
meaning ;  sometimes  it  refers  to  the  feeling  which  we  ex- 
perience, and  sometimes  to  that  property  or  condition  of 
the  external  body  that  causes  that  feeling.  In  the  former 
sense  it  is  true  that  there  is  no  heat  in  fire. 

The  word  warmth  is  more  exclusively  confined  to  the 
sensation,  and  therefore  we  can  say  in  a  more  unqualified 
manner  that  there  is  no  warmth — meaning  no  sensation  of 
warmth — in  fire.  It  is  true,  we  sometimes  speak  of  water 
being  warm,  and  it  is  perfectly  right  so  to  speak;  only 
when  we  do  so  it  is  important,  in  a  scientific  sense,  to  un- 
derstand that  what  we  mean  is  that  the  water  is  in  such  a 
condition  in  respect  to  temperature  as  to  create  the  sensa- 
tion of  warmth  in  us  when  we  put  our  hands  into  it,  not 
that  it  feels  any  sensation  of  warmth  in  itself. 

It  is  plain  that  there  can  be  no  sensation  of  any  kind  in 
the  fire  or  in  the  water,  an  idea  which  is  quaintly  expressed 
in  the  well-known  distich — 

"There's  no  warmth  in  the  fire  that  heats  yon, 
Than  there's  ache  in  the  stick  that  beats  you." 

In  the  same  manner,  it  is  true  that  there  is  no  sound  in 


TEE   6AUDKNS  OV   TbE   TUILKEIE3. 


GOING   TO    BREAKFAST.  209 

the  bell.  There  are  vibrations  in  the  bell  which  produce 
the  sensation  of  sound  in  us,  but  no  such  sensation  can  ex- 
ist in  the  bell. 

Another  very  striking  illusion  may  be  created  by  the 
sense  of  feeling.  If  you  cross  your  middle  finger  over  the 
fore  finger,  and  place  a  pea,  or  any  other  small  round  ob- 
ject, between  the  ends  thus  crossed,  and  roll  it  between 
them  in  the  palm  of  your  hand,  you  have  a  sensation  of 
two  peas,  especially  if  you  shut  your  eyes.  The  illusion  is 
very  strong  if  you  perform  the  experiment  upon  another 
person — a  little  child,  for  example — not  letting  him  know 
beforehand  how  many  peas  there  are.  Indeed,  the  effect  is 
produced,  though  not  so  strikingly,  by  feeling  of  any  small 
object,  as  the  end  of  your  little  finger,  with  the  two  fingers 
crossed  as  above  described. 

No  one  will  have  any  difficulty  in  admitting  that  these 
sensations  are  illusory,  but  I  do  not  expect  the  reader  will 
see  quite  so  easily  how  our  senses  deceiva  us  ir.  the  other 
cases  of  illusion  that  I  have  named,  such  as  that  of  the  col- 
ors in  nature,  the  rainbow  on  the  cloud,  and  the  arch  in  the 
sky. 

Lawrence  was  talking  on  this  subject  one  day  in  Paris 
with  John  when  they  wrere  on  their  way  to  breakfast, 
about  twelve  o'clock.  In  France  the  midday  meal,  which 
in  the  cities  in  America  is  known  as  luncheon,  is  called 
breakfast.  They  have  dinner  there  at  from  five  to  seven 
in  the  afternoon.  It  is  true  that  they  generally  take  a  cup 
of  coffee  and  a  roll  early  in  the  morning,  when  they  first 
rise,  but  they  call  this  simply  "  taking  coffee."  The  regu- 
lar breakfast  comes  about  noon.  Lawrence  and  John  had 
taken  coffee  that  morning  at  their  lodging  and  now  were 
going  to  a  restaurant  for  breakfast,  and  their  way  took 
them  across  the  garden  of  the  Tuileries,  which  are  beauti- 
ful public  gardens  in  front  of  the  palace  of  the  Tuileries, 


210  FLIPPY. 

and  are  frequented  by  great  numbers  of  people  every  pleas- 
ant day.  There  are  broad  and  handsome  walks,  and  groves 
of  trees,  and  seats,  and  smooth  open  spaces  where  children 
play — the  children  being  usually  good-natured  and  very 
polite  to  each  other,  as  is  the  custom  in  France. 

Lawrence  and  John  were  seated  together  upon  chairs 
near  a  massive  group  of  statuary  in  these  gardens,  talking 
together  on  the  subject  of  optical  illusions,  and  Lawrence 
had  already  said  to  John  what  has  been  stated  in  this  chap- 
ter, when  John's  eyes  accidentally  fell  upon  a  group  con- 
sisting of  a  gentleman  and  lady,  with  an  elegantly  dressed 
boy  accompanying  them,  who  were  walking  at  a  little  dis- 
tance. John  did  not  recognize  the  boy  as  any  person  that 
he  had  ever  seen  before ;  indeed,  he  did  not  pay  particular 
attention  to  him,  as  his  mind  was  occupied  with  listening 
to  what  Lawrence  was  saying,  when  all  at  once  the  boy 
suddenly  started  and  came  running  toward  the  seat  where 
Lawrence  and  John  were  sitting,  waving  his  cap  and  call- 
ing out,  Sac  a  papier!  Vive  lajoie!  He  paid  no  atten- 
tion to  his  father,  who,  in  very  earnest  and  authoritative 
voice,  was  calling  upon  him  to  stop  and  come  back.  John 
did  not  recognize  him  at  first,  but  he  soon  saw  that  it  was 
Flippy.  Those  who  have  read  the  volume  of  this  series 
entitled  HEAT  will  remember  Flippy  as  one  of  John's  fel- 
low-passengers in  crossing  the  Atlantic. 

The  shouts  that  Flippy  uttered  were  French  exclama- 
tions, common  among  French  boys  on  such  occasions,  the 
first  being  expressive  of  surprise,  and  the  other  of  exulta- 
tion ;  but  how  the  phrase  JBag  for  paper  !  has  ever  come 
1  to  be  used  for  an  expression  of  surprise  would  puzzle  the 
most  learned  philologists,  one  would  think,  to  determine. 
Flippy  was  beginning  to  learn  French,  and  such  expres- 
sions as  these,  when  he  heard  them,  made  a  great  impres- 
sion on  his  fancy,  and  he  used  them  on  every  occasion. 


BREAKFAST   PAETY    MADE    UP.  211 

FJippy's  father  and  mother,  as  soon  as  they  perceived 
that  it  was  Lawrence  and  John,  their  old  fellow-passen- 
gers, that  their  boy  had  discovered,  came  to  the  place,  and 
seemed  much  pleased  to  meet  them  again.  After  con- 
versing with  them  a  few  minutes,  they  took  their  leave, 
saying  at  the  same  time,  "  Come,  Flippy." 

But  Flippy,  who  was  not  under  very  good  government, 
remained  on  his  seat,  saying, "  No,  I  am  going  to  stay  with 
John." 

"  But,  Flippy,"  said  his  mother,  remonstrating,"  it  is  time 
for  us  to  go  to  breakfast." 

"  Have  you  had  breakfast,  John  ?"  said  Flippy,  turning 
to  John. 

"  No,"  said  John ;  "  we  are  going  now." 

"  Then  I'm  going  with  you,"  said  Flippy.  "  Mother,  you 
and  father  can  go  along ;  I  am  going  with  Mr.  Lawrence 
and  John." 

Flippy's  father  smiled;  he  seemed  to  look  upon  a  disa- 
greement of  this  kind  between  Flippy  and  his  mother  as 
an  amusing  contest,  in  which  there  was  no  occasion  for  him 
to  interfere. 

"But,  Flippy,"  said  his  mother,  in  an  expostulating  tone, 
"  Mr.  Lawrence  does  not  want  you ;  he  and  John  have 
plans  of  their  own." 

"  Yes,  he  does  want  me,"  said  Flippy.  "  Don't  you,  Mr. 
Lawrence  ?" 

"I  don't  like  to  have  you  disobey  your  mother,"  said 
Lawrence ;  "  but,  so  far  as  John  and  I  are  concerned,  we 
should  like  to  have  you  go  with  us  very  much ;  and,  if  you 
have  no  serious  objection,  Mrs.  Gray,  we  wish  you  would 
give  him  leave.  Have  you  any  ?" 

"  Oh  no  !"  said  Mrs.  Gray ;  "  I  have  no  objection ;  only 
you  will  find  him  very  much  in  your  way,  he  is  such  a 
heedless  and  troublesome  boy.  I  don't  think  he  really 


212  FLIPPY. 

means  to  be  disobedient ;  indeed,  he  usually  obeys  mo  vei  y 
well — when  he  has  a  little  time  to  think." 

So  it  was  all  arranged,  and  Mr.  and  Mrs.  Gray,  leaving 
Flippy  with  Lawrence  and  John,  went  away. 

Pretty  soon  Lawrence  and  the  two  boys  rose  from  their 
seats  and  began  to  walk  slowly  along  on  their  way  to  the 
Palais  Royal,*  where  they  were  to  have  their  breakfast. 
As  soon  as  they  commenced  their  walk,  John  asked  Law- 
rence to  go  on  with  what  he  had  begun  to  say  about  illu- 
sions. 

"  He  is  showing  me  that  there  is  no  green  in  the  grass," 
said  John. 

"He  can't  make  me  believe  that,"  said  Flippy;  "I know 
there  is  green  in  the  grass,  for  I  can  see  it.  Look  there !" 
he  added,  pointing  triumphantly  to  a  beautiful  green  grass- 
plot  which  they  were  just  passing. 

Now  Flippy  was  perfectly  right  in  saying  that  he  could 
see  the  green  color  of  the  grass.  We  all  see  it.  But  the 
question  is,  what  is  the  precise  meaning,  in  a  philosophical 
sense,  of  seeing  any  thing.  It  means  that  a  sensation  is 
produced  in  our  minds  through  the  organ  of  the  eye — a 
sensation,  namely,  of  color — the  cause  of  which  is  in  the 
outward  object,  while  there  is,  however,  no  sensation  of 
color  in  the  outward  object  itself,  but  only  a  mysterious 
something  which  causes  the  sensation  in  us. 

"Do  you  think  there  is  any  prick,  such  as  you  feel,  in 
the  point  of  a  pin  ?"  asked  Lawrence. 

"  It  pricks  me,  at  any  rate,"  said  Flippy. 

"Yes,"  replied  Lawrence ;  "or,  as  we  say  philosophically, 
it  causes  a  pricking  sensation  in  you,  but  there  is  no  such 
sensation  in  the  point  of  the  pin ;  there  is  only  that  in  it 
which  produces  the  sensation  in  you.  It  is  much  the  same 
with  what  we  see.  Greenness,  as  a  sensation  or  perception, 
*  Pronounced  Pallay  llwoyall. 


ARRIVAL   AT  THE    RESTAURANT.  213 

is  in  us.  The  cause  of  it  is  in  the  grass,  but  there  is  no 
sensation  of  it  there." 

"  I  don't  understand  it  very  well,"  said  Flippy, "  but  all 
I  know  is,  that  I'm  sure  the  grass  is  green  and  the  sky  is 
blue." 

"How  is  it  about  the  image  in  a  looking-glass?"  asked 
Lawrence ;  "  do  you  think  there  is  any  thing  really  there, 
when  you  think  you  see  your  face  behind  it  ?" 

"  Why — no  !"  said  Flippy,  reflecting  a  moment ;  "  but 
that's  a  different  thing ;  besides,"  said  he,  "  there  must  be 
an  image  somewhere  or  other,  and  somehow  or  other  there, 
for  I  see  it." 

Flippy's  reply  was  not  very  consistent  with  itself,  it 
must  be  admitted,  as  those  whose  convictions  are  con- 
trolled altogether  by  appearances,  and  by  impressions  made 
upon  the  senses  and  the  imagination,  and  not  rectified  by 
reason,  are  very  apt  to  be  inconsistent.  Truth  is  always 
consistent  with  itself,  but  error  never. 

After  some  farther  conversation  of  this  kind,  the  party 
reached  the  restaurant  at  the  Palais  Royal  where  they 
were  to  take  their  breakfast,  and  they  were  so  much  occu- 
pied with  the  scenes  and  incidents  which  attracted  their 
attention  there  that  they  said  no  more  and  thought  no 
more  of  the  subject  of  illusions  at  that  time.  Flippy,  how- 
ever, was  not  at  all  to  blame  for  being  so  entirely  under 
the  dominion  of  his  senses  in  respect  to  his  ideas  of  the 
real  character  of  the  phenomena  that  manifested  them- 
selves around  him.  He  was  very  young,  and,  though  his 
senses  were  in  complete  and  perfect  operation,  his  reason 
was  yet  only  partially  developed.  It  is  only  slowly,  and 
by  a  gradual  advance  toward  maturity,  that  the  thinking 
and  reasoning  faculties  become  strong  enough  to  assert 
their  power,  and  to  enable  us  to  distinguish  between  what 
is  apparent  and  what  is  real.  A  little  child  thinks  the 


214  FLIPPY. 

rocks  and  trees  are  actually  moving  when  for  the  first  time 
he  passes  rapidly  in  a  steamer  along  the  banks  of  a  river. 
He  can  hardly  be  persuaded  that  they  do  not  move.  The 
images  of  them  do  really  move  among  each  other  on  the  re- 
tina of  his  eye,%and  he  thinks  that  the  objects  themselves 
must  move.  If  you  whirl  a  burning  stick  in  the  air  before 
him,  he  sees  a  ring  of  fire,  and  he  thinks  there  is  a  real  ring 
of  fire  there,  if  he  is  very  young.  There  is  a  real  ring  of 
the  color  of  fire  on  the  retina  of  his  eye,  and  he  thinks  there 
must  be  a  real  ring  conforming  to  it  in  the  air. 

When  he  grows  a  little  older,  he  understands  that  in 
these  simple  cases  the  appearances  do  not  correspond  with 
the  reality;  but  other  illusions  remain,  and  are  only  one 
after  another  slowly  discovered  to  be  such,  as  his  knowl- 
edge increases  and  his  reasoning  powers  become  gradually 
unfolded.  I  presume  there  are  many  readers  of  this  book 
whom  it  will  be  hard  to  convince  of  the  illusory  nature  of 
the  deceptive  appearances  described  in  the  next  chapter. 


THE   AUCH   OF  THE   SKY.  215 


CHAPTER  XXIV.     t 

ILLUSIONS    EXPLAINED. 

THERE  are  few  persons  whose  ideas  of  the  reality,  in  re- 
spect to  external  nature,  are  not  still  so  far  under  subjec- 
tion to  the  impressions  of  the  senses  that  they  are  not 
easily  to  be  convinced  that  the  arched  appearance  of  the 
sky  is  an  illusion. 

In  conversing  on  the  subject  with  John  a  few  days  after 
the  breakfast  with  Flippy  in  the  Palais  Royal,  Lawrence 
reasoned  in  this  way : 

"  The  arch  in  the  sky,  that  looks  so  much  like  a  reality, 
seems  to  come  down  to  the  ground  at  a  distance  of  per- 
haps four  or  five  miles  from  us." 

John  admitted  this,  only  he  had  always  thought  it  was 
farther  than  five  miles  to  the  place  where  the  sky  seemed 
to  come  down  to  the  ground. 

"It  makes  no  difference,"  said  Lawrence,  "what  we  sup- 
pose the  distance  to  be.  Call  it  ten  miles,  if  you  please. 
Whatever  the  distance  is,  if  we  go  to  that  place  we  shall 
find  the  sky  as  high  there  as  it  is  here.  Thus,  wherever 
we  are,  we  have  a  sky  over  our  heads  as  high  and  as  arched 
in  one  place  as  in  another.  If  there  were  any  thing  real 
in  this  arched  appearance,  the  whole  surface  of  the  globe 
would  be  covered  with  domes,  like  inverted  cups,  cutting 
each  other  in  every  conceivable  way.  This  idea  is  evi- 
dently absurd.  The  truth  is,  that  the  dome-like  form  of 
the  sky  is  an  illusion.  It  results  from  certain  laws  in  re- 
spect to  the  motion  of  light,  and  the  effect  which  is  pro- 
duced upon  our  sense  of  vision  by  rays  coming  from  dif 


210  ILLUSIONS    EXPLAINED. 

ferent  distances  and  in  different  directions,  so  that  the  im- 
age of  a  dome  is  formed  on  the  retina  of  our  eyes  when 
there  is  nothing  in  external  nature  to  conform  to  it." 

Thus  the  vaulted  appearance  of  the  sky  is  the  creation 
of  our  senses,  qr,  rather,  of  our  minds  under  the  illusive 
evidence  of  our  senses.  The  vault  forms  itself  over  our 
heads  wherever  we  are,  and  we  carry  it  with  us  wherever 
we  go.  Each  person  has  his  own  sky,  corresponding. to 
his  own  position,  wherever  it  is,  and  it  is  a  different  one 
from  that  of  any  person  who  is  in  any  different  position. 
There  may  be  many  objects  common  to  both,  and  those 
which  are  at  a  great  distance  may  be  very  neai'ly  in  the 
same  relative  position,  but  they  are  really  different ;  so 
that,  wherever  we  go,  our  senses  form  for  us  continually 
an  ever-changing  sky  over  our  heads,  in  which  the  objects 
appearing  in  it  that  are  comparatively  near,  such  as  the 
clouds,  kites  flying,  birds,  and  the  outlines  of  distant 
mountains,  or  the  summits  of  spires  tall  enough  to  appear 
in  the  sky,  as  we  move,  continually  change  their  relative 
positions,  and  some  of  them  finally  disappear,  while  other 
objects  come  into  view  to  take  their  places.  In  respect  to 
these  various  objects,  there  is  for  each  a  reality  which  pro- 
duces the  image  of  it  in  our  minds ;  but  as  to  the  vaulted 
appearance  of  the  form  which  the  assemblage  of  them  as- 
sumes, it  is  all  an  illusion. 

The  nature  of  the  illusion  is  partly  explained  by  the  fact 
that  objects  appear  smaller  at  a  distance  than  when  near. 
Thus,  of  two  ships,  the  mast  of  one  which  is  near  appears 
much  taller  than  the  one  which  is  at  a  distance — that  is, 
the  top  mavks  a  much  higher  point  in  the  sky,  though  the 
one  may  be  really  no  taller  than  the  other. 

We  see  this  plainly  illustrated  by  the  masts  of  the  two 
whale  ships  in  the  engraving  on  the  opposite  page. 

It  results  from  this  principle  that,  in  the  case  of  an  ob- 


TOPS    OF   MASTS, 


217 


MASTS  OF   NEAR   AND   DISTANT   SHIPS. 


ject  approaching  us  from  a  distance,  the  top  of  it,  while  it 
really  remains  always  at  the  same  level,  appears  to  rise. 
This  is  the  explanation  of  the  apparent  rising  of  the  clouds 
in  the  western  sky  on  a  summer's  day,  and,  in  part,  of  their 
increasing  magnitude  as  they  draw  near.  It  is  true  that 
clouds  may,  and  often  do,  rise  and  descend  in  some  degree 
as  ascending  or  descending  currents  in  the  air,  when  such 
happen  to  exist,  may  chance  to  waft  them.  But  any  real 
changes  of  elevation  produced  in  this  way  are  very  small 
and  insignificant  compared  with  the  immense  apparent 
ascension  of  the  clouds  as  they  advance  from  the  horizon 
to  our  zenith.  The  whole  of  that,  substantially,  is  a  mere 
illusion. 

The  nature  of  the  effect  is  shown  clearly  when  we  see  a 

flock  of  birds  approaching  us  in  a  long  line.    Those  which 

are  near  us  look  far  higher,  when  we  regard  their  apparent 

positions  as  points  projected  against  the  sky,  than  those 

K  ~ 


218 


ILLUSIONS    EXPLAINED. 


which  are  remote.  When  the  objects  are  birds  (and  we 
know  that,  since  they  belong  to  the  same  flock,  and  are 
consequently  of  the  same  species,  they  must  be  of  substan- 
tially the  same  size),  the  difference  of  apparent  size  sug- 


gests at  once  to  our  minds  the  difference  of  distance,  and 
so  enables  us  unconsciously  to  correct  the  false  impression 
which  would  otherwise  be  produced ;  but  inasmuch  as,  in 
the  case  of  clouds,  of  the  real  magnitudes  of  which  AVC  have 
no  means  of  judging,  there  is  nothing  to  correct  the  sensi- 
ble impression,  their  apparent  rising,  though  an  illusion, 
has  the  full  force  of  reality  upon  our  minds. 

The  glittering  colors  which  we  think  we  see  in  the  drops 


THE    RAINBOW.  219 

of  dew,  which  make  us  imagine  that  one  is  green,  another 
rose-colored  or  orange,  and  another  blue,  are  also  illusions. 
Each  drop  separates  the  beam  of  light  which  strikes  upon 
it  into  its  component  hues,  or,  rather,  into  the  different  vi- 
brations, or  other  forms  of  force  by  which  the  conception 
of  these  different  hues  are  awakened  in  our  minds,  and 
sends  them  off  in  different  directions,  and  the  drop  that  we 
see  seems  to  be  of  the  color  of  the  particular  hue  which 
comes  from  it  to  our  eye.  If  we  move  our  eye  a  little  way 
we  come  into  a  position  when  a  different  portion  of  the 
spectrum  falls  upon  it,  and  the  drop  which  a  moment  be- 
fore looked  red  now  looks  green.  Of  course,  of  different 
persons  looking  at  different  drops,  no  two  would  see  the 
same  colors  in  the  same  drops.  A  drop  that  would  send 
a  blue  ray  to  one  would  send  a  yellow  ray  to  another,  and 
to  a  third  perhaps  no  light  at  all. 

Of  course,  when  I  speak  of  a  ray  of  any  specified  color,  I 
mean  a  ray  having  the  power  to  produce  the  sensation  of 
that  particular  color  in  our  minds. 

The  case  is  somewhat  analogous  to  this  in  the  phenom- 
enon of  the  rainbow.  When  there  is  a  dark  cloud  consist- 
ing of  falling  drops  of  rain  in  the  east,  and  the  sun  comes 
out  bright  in  the  west,  the  rays,  striking  the  drops,  are 
turned  back  by  reflections  and  refractions,  and  are  sepa- 
rated into  their  component  parts  precisely  as  they  are  by 
similar  drops  of  water  in  the  dew.  These  different  classes 
of  rays  are  sent  off  from  every  falling  drop  in  every  possi- 
ble direction,  so  that  the  whole  atmosphere — all  around  us, 
and  through  the  whole  space  between  us  and  the  cloud — 
is  filled  with  these  innumerable  radiations,  crossing  each 
other  in  every  conceivable  manner,  but  without  any  sensi- 
ble interference,  or  interruption  of  one  by  the  other,  or 
the  least  confusion  in  the  sensations  of  color  which  these 
immensely  varied  vibrations,  if  they  are  really  vibrations, 


220  ILLUSIONS   EXPLAINED. 

are  capable  of  exciting  in  our  minds.  What  takes  place  at 
such  a  time  in  the  action  of  the  sun's  rays  upon  the  drops 
of  falling  rain  constitutes  one  of  the  most  wonderful  phe- 
nomena in  nature — one  the  possibility  of  which  would  be 
utterly  inconceivable  by  us  if  there  were  not  the  most  ir 
refragable  proof  of  the  reality  of  it. 

Now  the  manner  in  which  these  descending  drops  sepa- 
rate the  sun's  rays  falling  upon  them,  and  send  the  differ- 
ent portions  radiating  in  various  directions  in  such  a  man- 
ner as  to  form  upon  the  retina  of  the  eye  of  any  person 
looking  at  the  cloud  the  image  of  a  rainbow,  is  somewhat 
difficult  to  be  explained.  Sometimes  a  picture  of  a  rain- 
bow is  made,  with  lines  to  represent  the  course  of  the  rays 
drawn  from  it  to  the  eye  of  an  observer  on  the  ground, 
who  is  also  represented  in  the  picture.  But  such  a  diagram 
tends  rather  to  confuse  the  ideas  of  the  student  than  to  aid 
him,  first,  because  it  represents  the  bow  on  the  cloud  as  a 
reality,  when  there  is  no  such  reality  there,  and,  secondly, 
because  the  image  of  it  in  the  eye  could  never  appear  to 
be  in  the  same  position  for  the  observer  shown  in  the  land- 
scape and  for  the  person  looking  at  the  picture.  The  rain- 
bow is  sometimes  even  represented  as  foreshortened  by 
perspective,  as  if  it  were  a  solid  arch  of  many  colors  built 
into  the  sky.  Now  the  idea  of  a  rainbow  foreshortened,  as 
if  seen  obliquely,  if  not  involving  a  contradiction  in  terms, 
is  certainly  a  philosophical  absurdity. 

I  shall  therefore  endeavor  to  give  the  reader  some  idea 
of  the  general  principle  on  which  the  rainbow  image  is 
formed  in  the  eye  by  means  of  the  drops  of  falling  rain, 
without  any  engraving  to  illustrate  it,  though  I  have  sev- 
eral at  hand  made  expressly  for  the  purpose.  I  only  ask 
the  reader  to  imagine  himself  to  be  looking  out  at  the  door 
toward  the  east  on  a  summer  afternoon,  just  after  a  shower 
has  passed  over,  and  the  cloud  lies  in  the  eastern  sky,  while 


BI1FLEOTIOS   FEOM   THE   IMSE 


REFRACTION    AND  REFLECTION    IN    THE    DROPS.         223 

the  sun  has  just  come  into  view,  breaking  through  the 
clouds  in  the  west. 

In  the  shower-cloud  in  the  east  the  rain  is  still  falling ; 
the  sky  in  that  quarter  is  full,  in  fact,  of  falling  drops.  As 
the  rays  of  the  sun  enter  these  drops,  they  are  at  first  re- 
fracted at  the  surface  where  they  enter,  and  are  partially 
separated  into  their  component  portions.  When  they  reach 
the  back  side  of  the  drop,  some  of  them  strike  it  at  such  an 
angle  that  they  are  reflected  by  the  inner  side  of  that  sur- 
face, just  as  the  light  is  reflected  from  the  under  side  of  the 
upper  surface  of  water  in  a  tumbler,  as  has  already  been 
explained.  The  reflected  light,  then  passing  out  on  the 
front  part  of  the  drop — that  is,  on  the  side  toward  the  sun, 
the  same  by  which  it  entered — is  there  refracted  more,  and 
the  rays  then  come  back  toward  the  west  again,  those  char- 
acteristic of  the  various  colors  being  separated  from  each 
other,  and  diverging  more  and  more  widely  as  they  pass 
through  the  air. 

Now  one  would  suppose  that,  this  being  the  state  of 
the  case,  the  whole  air  Avould  be  filled  with  these  radiating 
colors,  or,  rather,  with  the  different  radiations  capable  of 
producing  these  different  colors,  and  that  is  the  fact.  But 
it  is  proved  by  the  most  exact  and  profound  mathematical 
calculations  that  any  individual  drop  must  be  in  a  certain 
precise  and  determinate  position  in  reference  to  the  ob- 
server in  order  to  send  the  rays  of  any  particular  color— 
the  green,  for  example — to  his  eye ;  and  that  all  the  drops 
which  are  in  the  same  relative  position  will  send  the  same 
rays  to  him ;  and  those  which  are  in  the  same  relative  po- 
sition are  those  which  come  in  the  range  of  a  circle  of 
which  the  point  opposite  to  his  eye,  in  relation  to  the  sun, 
is  the  centre — that  is,  the  point  determined  by  a  line  drawn 
from  the  sun,  through  the  position  of  his  eye,  to  the  cloud. 
Thus,  if  a  drop  at  a  certain  distance  above  that  point  sends 


224  ILLUSIONS   EXPLAINED. 

a  green  ray— or,  more  strictly,  a  green-producing  ray — to 
his  eye,  all  the  other  drops  in  the  range  of  a  great  circle 
drawn  at  the  same  distance  from  the  central  point  above 
'named  will  do  the  same.  But,  as  the  sun  is  above  the  ho- 
rizon, the  central  point  of  this  circle,  which  is  the  point  ex- 
actly opposite  to  the  sun,  must  be  below  it,  and,  of  course, 
less  than  half  the  circle  will  come  upon  the  sky;  but  all 
the  drops  which  come  at  the  same  distance  from  the  centre 
in  that  portion  of  the  circle  which  has  rain-drops  in  it  will 
send  green  rays  to  the  eye. 

Now  it  is  evident  that  if  an  eye  in  any  given  position  is 
in  the  range  of  the  green  rays  from  any  drop  in  the  cloud, 
it  would  be  out  of  the  range  of  its  yellow  or  its  blue  rays, 
and  that  the  drops  which  would  send  these  rays  to  his  eye 
would  be  for  the  one  color  above,  and  for  the  other  below 
the  one  which  sent  him  green-producing  light.  Each  of 
these,  too,  would  have  its  circular  band  of  drops  formed 
of  all  those  that  were  at  the  same  distance  from  the  cen- 
tral point  with  themselves,  all  of  which  wrould  take  effect 
in  sending  those  component  parts  of  the  solar  ray  to  the 
eye,  which  would  produce  there  the  sensations  of  their 
color.  Thus,  while  the  whole  sky  is  filled  with  falling 
drops,  each  of  which  is  sending  the  same  diversified  radia- 
tions in  precisely  the  same  manner,  only  those  radiations 
of  any  particular  color  would  reach  the  eye  of  an  observer 
in  any  one  place  which  came  from  drops  within  the  range 
of  a  circle  drawn  at  a  certain  distance  from  the  point  op- 
posite to  his  eye,  and  thus  would  be  produced  the  appear- 
ance of  concentric  colored  bands. 

Of  course,  if  he  moves  his  position,  he  brings  himself 
into  a  corresponding  range  of  radiation  from  a  new  set  of 
drops,  and  so  changes  the  position  of  the  rainbow  in  the 
cloud.  In  other  words,  he  forms  the  rainbow  for  himself, 
wherever  he  stands,  out  of  the  light  coming  from  the  drops 


NATURE    OP   SENSATIONS.  225 

which  are  in  the  right  position  in  relation  to  the  place  of 
his  eye.  As  he  moves,  he  carries  the  apparent  place  of  the 
rainbow  with  him,  and  two  persons  standing  side  by  side 
see  different  bows  in  this  sense,  namely,  that  the  rays  that 
produce  the  image  in  their  organs  of  vision  respectively  do 
not  come  from  the  same  drops  in  the  sky,  and  the  bows  are 
not  seen  in  the  same  position.  It  would  perhaps  be  too 
much  to  say  that  they  do  not  see  the  same  bow  in  any 
sense,  as  the  question  in  what  the  identity  of  a  rainbow 
really  consists  is  one  which  might  very  naturally  give  rise 
to  great  difference  of  opinion. 

It  is,  however,  at  any  rate,  perfectly  sure  that  the  rain- 
bow is  not  an  object  real  and  fixed  upon  the  cloud — one 
which  we  look  at  as  we  do  at  the  cloud  itself,  or  at  the 
moon,  or  a  star.  As  a  real  existence,  having  the  form  and 
appearance  which  it  presents  to  our  vision,  it  is  an  illusion. 
It  exists  only  in  the  eye  of  the  one  who  looks  upon  it. 

Thus  we  see  that  many  things  which  we  are  apt  to  con- 
ceive of  external  objects  having  a  real  existence,  or  as 
qualities  of  external  objects,  are,  in  reality,  ideas  or  sensa- 
tions in  us.  This  truth  applies  to  the  impressions  of  the 
other  senses,  such  as  the  hearing,  the  taste,  and  the  smell, 
as  well  as  to  the  sight.  The  words  sweetness  and  saltness, 
for  example,  denote  sensations,  and,  of  course,  there  can  be 
no  sensation  of  any  kind  in  sugar  or  salt.  There  can  only 
be  that  in  them  which  excites  these  sensations  on  the 
tongue  which  tastes  them. 

It  is  evidently  so  in  regard  to  all  the  impressions  made 
upon  our  senses.  External  objects  communicate  some 
form  of  force  to  our  organs  by  which  certain  sensations 
are  awakened  in  our  minds,  but  these  sensations  do  not 
and  can  not  exist  in  the  objects  themselves. 

Understood  in  this  way,  it  is  obviously  true,  as  Law- 
rence said,  that  there  is  no  image  in  the  mirror,  no  bow  in 
K2 


220  ILLUSIONS   EXPLAINED.' 

the  cloud,  no  arch  in  the  sky,  no  green  or  gold  in  the  dew, 
no  color  in  the  grass  or  the  flowers,  no  sweetness  in  sugar, 
no  fragrance  in  the  rose,  no  sound  in  the  bell,  and  no 
warmth  in  the  fire,  but  only  phenomena  taking  place  in 
the  external  objects  which  have  power  to  cause  those  sen- 
sations in  a  living  being. 

The  conversation  between  Lawrence  and  John,  which 
was  interrupted  by  the  dinner  at  the  Palais  Royal,  was  re- 
sumed after  dinner.  Flippy  was  quite  interested  in  such 
portions  of  it  as  he  could  understand,  and  that  evening  at 
tea  he  put  his  knowledge  to  the  very  questionable  use  of 
playing  a  joke  upon  his  mother.  Just  as  they  were  ready 
to  leave  the  table,  he  took  some  sugar  out  of  the  sugar- 
bowl  with  a  spoon,  and,  wetting  his  finger,  touched  a  little 
to  his  tongue. 

"Why,  mother,"  said  he,  "there's  no  taste  in  this  sugar!" 

"No  taste  !"  repeated  his  mother,  surprised. 

"  No,  mother,"  said  he ;  then  he  tasted  it  again. 

"  Let  me  see,"  said  his  mother ;  and,  taking  the  spoon 
from  his  hand,  she  tasted  it  herself,  very  daintily,  as  if  she 
expected  that  it  would  taste  like  salt.  She  found,  how- 
ever, that  it  was  good,  sweet  sugar. 

"  Why,  what  do  you  mean,  Flippy  ?"  she  said.  "  This 
sugar  is  all  right;  there  is  as  much  taste  in  it  as  in  any 
sugar." 

"  No,  mother,"  said  Flippy, "  the  taste  is  not  in  the  sugai-, 
it  is  all  in  your  tongue." 

So  saying,  Flippy  seized  his  cap  and  ran  off,  leaving  his 
mother  half  vexed  with  his  having  played  a  joke  upon  her, 
and  half  pleased  with  his  ingenuity  and  fun. 

"  Some  of  the  nonsense  he  has  got  from  Mr.  Wollaston," 
she  said,  turning  to  Mr.  Gray  with  a  smile,  "  I'll  engage." 


REFLECTION    AND    REFRACTION.  227 


CHAPTER  XXV. 

FORMATION    OF    IMAGES. 

LIGHT  tends  always  to  move  in  right  lines  in  proceeding 
from  its  source.  There  are,  however,  two  very  striking  and 
marked  modes  by  which  it  is  deflected  or  turned  from  its 
course.  The  first  is  called  reflection,  and  the  second  re- 
fraction. 

When  the  light,  in  its  progress,  falls  upon  a  liquid  or  a 
solid  surface  that  is  opaque,  a  portion  of  it — sometimes  a 
very  large  portion  of  it — rebounds,  as  it  were.  This  is 
called  reflection.  When  it  falls  upon  any  transparent  liq- 
uid or  solid,  it  passes  in ;  but,  except  when  it  enters  per- 
pendicularly, it  is  bent  somewhat  out  of  its  direct  course 
in  entering,  and  also  again  in  emerging  on  the  farther  side, 
if  it  does  emerge.  Thus  we  may  say,  in  general  terms,  that 

Reflection  is  the  turning  back  of  a  ray  of  light,  or  a  por- 
tion of  a  ray,  in  falling  upon  a  surface  which  does  not  al- 
low it,  or  the  whole  of  it,  to  pass  through  ;  while 

Refraction  is  the  bending  of  a  ray  of  light,  or  a  portion 
of  it,  in  entering  and  leaving  a  substance  which  does  allow 
it,  or  a  part  of  it,  to  pass  through. 

Probably  many  of  the  readers  of  this  book  may  have 
known  this  before,  and  they  may  also  know  some  other 
things  that  I  am  about  to  state,  especially  those  in  respect 
to  reflection.  Indeed,  some  of  these  facts  have  already 
been  referred  to  in  a  preceding  chapter.  But,  even  if  you 
know  certain  facts  individually  and  separately,  it  is  a  great 
advantage  to  have  them  brought  together  and  stated  in  a 
systematic  manner,  so  as  to  show  them  in  their  relations 


228  FORMATION    OF   IMAGES. 

to  each  other,  and  thus,  as  it  were,  to  connect  what  would 
otherwise  be  isolated  facts  into  one  systematic  and  harmo- 
nious whole. 

To  know  facts  separately,  without  any  understanding  of 
their  connection  with,  and  bearing  upon  each  other,  or  of 
the  general  principles  which  they  individually  exemplify, 
and  to  act  only  upon  knowledge  lying  in  that  form  in  the 
mind,  is  called  empiricism  ;  but  when  the  same  facts  are 
arranged  in  systematic  order,  so  that  their  relations  to 
each  other,  and  the  general  principles  which  they  severally 
exemplify,  are  brought  to  view,  the  knowledge  becomes 
scientific.  Accordingly,  in  what  I  am  about  to  state  in  re- 
spect to  reflection  is  intended  to  arrange  in  your  minds  in 
a  somewhat  scientific  manner  facts  most  of  which,  and  per- 
haps all  of  which,  you  already  know  in  an  empirical  man- 
ner. 

When  any  opaque  surface  is  plane,  smooth,  and  highly 
polished,  so  that  all  the  portions  of  it  on  which  the  light 
strikes  present  themselves  to  the  rays  at  the  same  angle, 
each  ray  is  reflected  in  the  same  manner — that  is,  accord- 
ing to  the  same  law,  and,  after  reflection,  they  all  proceed 
in  the  same  directions  in  relation  to  each  other^  as  before, 
though  in  relation  to  surrounding  objects  the  direction  of 
the  whole  beam  is  turned. 

The  consequence  is,  that  light  so  reflected  enters  the  eye 
precisely — in  respect  to  the  character  and  constitution  of 
the  beam — as  if  it  had  not  been  reflected,  only  it  comes 
from  another  direction  /  the  object  from  Avliich  it  comes 
of  course  appears  of  its  proper  form  and  size,  and  only 
seems  to  be  in  another  place. 

This  is  just  what  takes  place  when  we  see  any  thing  re- 
flected in  a  plane  mirror.  The  engraving  represents  the 
course  of  the  rays  of  light  in  such  a  case,  taking  those  em- 
anating from  one  point,  namely,  the  tip  of  the  flame,  for  an 


REFLECTION    FROM    A   MIKKOR.  229 


THE   TI.ANE  MIKKOR. 


example.  The  light  radiates  from  this  point,  of  conrse,  in 
all  directions,  though  only  the  portion  of  it  which  ultimate- 
ly reaches  the  eye  of  the  observer  is  represented  by  lines 
in  the  drawing.  This  portion  diverges  as  it  leaves  the 
point  of  the  flame  till  it  strikes  the  glass,  which  is  seen 
edgewise  in  the  centre  of  the  picture,  and  then,  after  reflec- 
tion, continues  to  diverge  just  as  before,  on  account  of  the 
fact  that  the  glass  being  plane,  every  ray  is  reflected  in 
the  same  manner,  and  the  whole  pencil  or  beam  continues 
its  course,  after  reflection,  without  change,  except  in  its 
general  direction ;  and  as  the  image  in  the  eye  of  the  ob- 
server is  determined  by  the  character  of  the  rays  as  they 
enter  his  eye,  the  tip  of  the  lamp  flame  will  appear  as  if  it 
was  situated  as  far  behind  the  glass  as  it  is  in  reality  be- 
fore it. 

It  is,  of  course,  the  same  with  every  other  point,  both  in 
the  flame  itself,  and  in  all  the  parts  of  the  candle  and  the 
candlestick,  although,  to  prevent  confusion,  those  from  only 
one  point  arc  represented  in  the  engraving,  and  of  those 
issuing  from  that  point,  only  that  small  portion  are  drawn 
which  ultimately  reach  the  observer's  eye. 

The  engraving  thus  fails  to  represent  the  facts  as  they 


230  FORMATION    OP    IMAGES. 

are  in  two  respects:  first,  it  shows  an  image  behind  the 
glass,  when  in  fact  there  is,  in  reality,  no  such  image  there ; 
what  is  meant  to  be  shown  is  that  the  rays  enter  the  eye 
just  as  if  there  were  such  an  image ;  and,  secondly,  the  il- 
lumination from  one  radiant  point  only  is  shown,  and  of 
those  only  that  portion  that  finally  enter  the  eye,  while  in 
reality  the  rays  proceed  in  every  direction,  both  from  thai 
point  and  from  every  other,  those  from  each  point  crossing 
those  from  every  other  point — every  where— and  in  mil- 
lions of  intersections,  which  it  would  be  impossible  to  rep- 
resent in  any  drawing.  Indeed,  the  case  presents  to  our 
conceptions  phenomena  so  marvelous,  that  if  we  consider 
every  ray  as  a  distinct  and  independent  undulation,  we  can 
hardly  picture  to  our  minds  such  a  maze  of  crossings  and 
interminglings,  made  without  disturbance  or  confusion,  as 
a  possibility. 

If,  instead  of  a  silvered  glass,  forming  a  proper  mirror,  a 
plate  of  plain  glass  were  to  be  used,  the  effect  would  be 
substantially  the  same.  It  would,  in  fact,  be  exactly  the 
same  if  proper  precautions  were  taken  to  prevent  the  trans^ 
mission  of  light  from  the  other  side  of  the  glass  to  mingle 
with  and  confuse  that  which  wras  reflected,  or,  rather,  to 
mingle  and  confuse  the  two  sets  of  images  which  they 
would  respectively  form  on  the  retina  of  the  eye.  The  sev- 
eral rays  of  light  would  not  confuse  or  disturb  each  other 
at  all  on  their  passage  through  the  air.  Each  would  pur- 
sue its  own  way  unimpeded  by  the  rest,  and  each  set  would 
form  its  own  image  on  the  retina.  It  is  only  the  mind  that 
would  be  confused  in  its  efforts  to  separate  and  distinguish 
the  images. 

If,  now,  the  reflecting  plate  of  glass,  instead  of  being 
plane  and  uniform  throughout  its  Avhole  surface,  were  to 
be  broken  up,  and  the  fragments  thrown  in  confusion  into 
a  basket,  it  is  evident,  as  was  briefly  explained  in  the  chap- 


WHITE    SUBSTANCES.  231 

ters  on  spectres  and  ghosts,  that  the  image  seen  in  the  frag- 
mentary surfaces  would  be  divided,  and  the  portions  va- 
riously dispersed;  for  the  broken  surfaces  of  the  glass, 
being  inclined  to  each  other,  would  form  different  angles 
with  the  rays  incident  upon  them,  and  would  reflect  them 
differently,  and  thus  a  mass  of  confused  and  disjointed 
gloamings  would  be  the  result. 

If,  instead  of  being  broken  into  fragments  of  sensible 
size,  the  glass  were  to  be  ground  into  a  fine  powder,  the 
particles  would  still  reflect  the  light  falling  upon  them,  but 
the  rays  would  be  mingled  in  inextricable  confusion,  and, 
instead  of  producing  any  distinct  images  in  the  eye,  or 
even  distinct  portions  of  images,  they  would  only  produce 
the  impression  of  a  general  white  light. 

This  is  supposed  to  be  the  manner  in  which  the  sensa- 
tion comes  to  us  from  any  white  substance.  The  compo- 
sition of  the  surface  is  such  that  it  takes  up  the  light  that 
falls  upon  it,  and  reflects  it  to  our  eyes  in  a  confused  med- 
ley of  beams  that  can  form  no  image  on  the  retina,  but 
only  produce  a  general  luminous  effect. 

If,  however,  we  contrive  by  artificial  means  to  smooth 
and  polish  any  white  surface,  as  that  of  marble,  for  in- 
stance, we  can  impart  to  it  the  power  of  faintly  and  indis- 
tinctly reflecting  an  image — that  is,  by  the  process  of  pol- 
ishing, we  form  among  the  particles  so  many  faces  lying  in 
the  same  plane,  that  by  their  combined  effect  they  throw  a 
sufficient  number  of  rays,  coming  from  any  object  near,  in 
a  regular  manner  to  the  eye,  to  form  an  image  distinct, 
perhaps,  in  its  general  features,  but  faint,  on  account  of 
these  rays  being  mingled  with  others  coming  in  confusion 
from  the  particles  which  are  not  in  the  same  plane. 

This,  then,  is  the  explanation  of  the  manner  in  which  the 
sensation  of  whiteness  is  formed  in  our  organs  of  vision. 
A  surface  that  appears  white  reflects  the  light  that  falls 


232  FOKMATIOX    OF   IMAGES. 

upon  it  in  a  confused  and  irregular  manner,  so  that  it  pro* 
duces  upon  the  retina  of  the  eye  no  image  regularly  re- 
flected, but  only  a  general  impression  of  light. 

But  some  surfaces,  which  have  such  a  constitution  in  re- 
spect to  the  disposition  of  their  particles  that  they  can  only 
reflect  the  light  that  falls  upon  them  in  a  confused  and  ir- 
regular manner,  seem  to  have  in  them  some  mysterious 
power  of  making  a  selection  among  the  rays  thus  foiling, 
and,  while  a  portion  are  reflected,  the  others  disappear.  It 
is  customary  to  say  that  they  are  absorbed — that  is  to  say, 
in  the  case  of  green  leaves  and  grass,  for  instance,  the  ordi- 
nary white  light  from  the  sun  falls  upon  them,  but  is  not 
reflected  as  ichite  light.  Of  the  seven  primary  colors  of 
which  the  white  light  is  composed,  all  but  the  green  are 
in  some  way  apparently  suppressed  or  extinguished.  The 
green  is  reflected,  and,  coming  to  our  eyes,  produces  there 
the  sensation  of  green.  So  we  say  the  grass  is  green. 

It  is,  of  course,  an  important  and  curious  question  what 
becomes  of  the  rays  which  disappear.  They  are  said  to 
be  absorbed — that  is,  that  they  enter  in  some  way  into  the 
leaves  or  the  grass,  and  remain  there.  All  light,  whether 
we  can  correctly  picture  it  to  our  minds  as  a  vibratory  or 
undulatory  motion  or  not,  is  undoubtedly  the  action  of 
some  form  or  some  kind  of  force,  and  the  prevailing  idea 
among  scientific  men  is  that  that  portion  of  this  force 
which  represents  green  is  turned  back  from  the  grass- 
blade,  or  the  leaf  of  the  tree,  into  the  air,  while  the  re- 
mainder enters  the  tissues  of  the  plant,  and  is  there  con- 
verted into  some  other  of  the  numerous  forms  of  hidden 
force  which  is  always  in  action  among  the  molecules  or 
atoms  of  all  substances,  and  on  which  the  properties  of  the 
substances  and  the  changes  which  they  undergo  depend. 

It  is  the  same  with  all  the  other  colored  substances  ex- 
isting in  nature  or  produced  by  art.  They  have  the  power 


THE    MAGIC   TELESCOPE.  233 

of  absorbing  all  of  the  light  which  falls  upon  them  except 
that  portion  which  forms  the  color  which  they  present  to 
the  eye.  And  this,  as  was  stated  at  the  close  of  the  last 
chapter,  is  the  philosophy  of  color. 

In  respect  to  the  regular  and  systematic  reflection  which 
is  produced  from  smooth  and  polished  surfaces,  if  the  sur- 
faces are  plane,  the  rays  are  reflected,  as  has  already  been 
explained,  in  such  a  manner  that  their  relative  condition 
in  respect  to  each  other  is  not  changed.  The  whole  beam 
is  turned  out  of  its  course,  it  is  true,  but  without  any 
change  in  its  internal  constitution,  and  the  image  which  it 
is  capable  of  producing  in  the  eye  is  not  changed  in  its 
form  or  in  its  magnitude,  but  only  in  its  apparent  place. 
When  the  reflecting  surface  is  not  plane,  but  is  of  some 
other  regular  mathematical  form,  as,  for  example,  when  it 
is  concave,  convex,  cylindrical,  or  conical,  the  rays  are  re- 
flected with  regularity,  so  as  to  form  images  on  the  retina 
of  the  eye;  but  these  images  are  enlarged,  or  diminished, 
or  changed  in  various  ways,  according  to  the  effect  of  the 
surface  in  modifying  the  directions  of  the  rays  in  respect 
to  each  other.  The  eye,  it  must  always  be  remembered, 
can  take  cognizance  of  the  rays  only  when  they  enter  it, 
and  is  wholly  unconscious  of  any  change  of  direction  which 
they  may  have  been  subjected  to  on  their  passage. 

There  is  a  very  curious  piece  of  apparatus,  called  the 
magic  telescope,  which  serves  admirably  to  illustrate  this 
principle. 

There  is  a  stand  with  a  concealed  channel  passing 
through  it,  in  which  small  square  pieces  of  looking-glass 
are  fixed  at  angles  of  45°,  one  at  each  end.  Above  these 
ends  are  two  upright  tubes  which  have  the  appearance 
only  of  simple  supports,  though  they  are  really  hollow. 
Upon  each  of  these  supports  are  two  short  tubes,  like  tele- 
scope tubes,  with  an  open  space  between  them.  In  each 


234  FORMATION    OF    IMAGES. 


SEEING   TIIEOUGn 


of  these  tubes  is  a  piece  of  looking-glass  like  those  below, 
and  they  are  placed  in  such  a  manner,  at  an  angle,  that 
light  from  any  object— a  candle,  for  instance,  or  a  finger, 
or  a  key — placed  in  front  of  one  of  the  tubes,  is  reflected 
down  into  the  channel  in  the  stand,  thence  along  the  chan- 
nel, and  up  through  the  other  tube  to  the  eye  of  the  ob- 
server. A  big  stone,  then,  or  any  other  object  perfectly 
opaque,  may  be  placed  in  the  open  space  between  the  two 
tubes,  and  a  person  looking  through  can  see  the  candle  or 
the  key  appai-ently  through  the  stone,  by  means  of  the 
light  which  is  carried  down  through  the  stand  by  the  re- 
flectors. The  eye  of  the  observer  takes  cognizance  of  the 
rays  as  they  enter  the  eye,  and  judges  of  the  position  of 
the  object  from  which  they  proceed  solely  from  the  direc- 
tion in  which  they  come  in  thus  entering. 

And  so  it  is  in  all  cases.  By  means  of  reflectors  of  va- 
rious mathematical  forms,  the  course  and  relative  direction 
of  any  rays  can  be  changed  in  almost  any  conceivable  man- 
ner, and  made  to  enter  the  eye  in  any  condition,  as  to  di- 
rection and  power,  that  the  experimenter  may  please;  and 
whatever  may  be  the  surface  which  reflects  any  light,  if  it 


VARIOUS   IMAGES. 


237 


is  of  a  regular  and  mathematical  form,  the  precise  effect 
which  it  will  produce  upon  the  rays,  and  the  condition  in 
which  they  will  enter  the  eye  after  reflection,  can  be  cal- 
culated, and  the  kind  of  image  which  will  be  formed  ascer- 
tained beforehand.  Sometimes  the  image  is  diminished, 
sometimes  it  is  magnified  ;  sometimes  it  is  multiplied,  and 
sometimes  it  is  reversed.  The  latter  takes  place  when  the 
rays  are  made  to  cross  each  other  before  they  form  the 
image  on  the  retina,  as  maybe  illustrated  by  the  image  of  a 
candle  formed  upon  a  screen  in  a  dark  room,  after  the  rays 
cross  each  other  in  passing  through  a  very  small  orifice. 

The  effects  produced  by  the  reflection  of  light  in  mirrors 
of  various  kinds,  in  respect  to  the  conformation  of  the  re- 


ENLAEGEMENT  IN   A   CONCAVE  MIEEOK. 


238  FORMATION    OF   IMAGES. 

fleeting  surface,  are  very  surprising  and  very  beautiful. 
They  are  all,  however,  the  subjects  of  very  exact,  though 
quite  intricate  mathematical  calculations.  If  the  mirror  is 
convex,  it  gives  a  diminished  image  of  the  object,  as  you 
see  when  looking  at  your  face  in  the  back  of  the  bowl  of  a 
bright  silver  spoon.  If  concave,  an  enlarged  image  when 
the  object  is  held  near,  but  a  diminutive  one  if  held  at  a 
greater  distance,  as  may  be  observed  in  the  inside  of  the 
bowl  of  the  spoon. 

If  the  mirror  is  cylindrical  or  conical,  the  image  is  very 
curiously  distorted ;  but  the  distortion  is  all  subject  to  ex- 
act calculation,  which  is  so  certain  in  its  character  that  the 
reverse  effect  can  be  produced  by  calculating  and  drawing 
a  distorted  picture  such  that  when  reflected  it  shall  conie 
true. 


Toys  are  often  made  on  this  principle,  as  shown  in  the 


THE   SCIENCE    OF   OPTICS.  239 

engraving,  where  the  drawing  on  the  card  below,  which 
would  seem  to  be  a  wholly  unmeaning  scrawl,  is  brought, 
by  reflection  in  a  cylindrical  mirror,  into  a  very  significant 
image. 

The  study  of  the  effects  produced  by  the  reflection  of 
light  from  the  various  geometrical  surfaces  forms  a  branch 
of  mathematical  science  of  a  very  difficult  and  complicated 
character. 


240  LAWS    OF   REFLECTION   AND    REFRACTION. 


CHAPTER  XXVI. 

LAWS    OF   REFLECTION    AND    REFRACTION. 

WHEN  a  ray  of  light  passes  from  the  air  through  any 
transparent  medium  which  is  denser  than  air,  as  glass  or 
water,  for  instance,  if  it  enters  and  leaves  the  denser  medi- 
um at  right  angles  to  the  surface,  its  course,  it  will  be  rec- 
ollected, is  not  changed  in  its  passage,  but  if  it  enters  or 
leaves  at  any  oblique  angle,  it  is  turned  somewhat  from  a 
straight  course  both  in  entering  and  leaving. 

Now  we  have  seen  in  the  case  of  reflection  that,  inas- 
much as  the  direction  in  which  a  ray  is  reflected  depends 
upon  the  angle  at  lohich  it  strikes  the  reflected  surface,  it  fol- 
lows that  when  the  surface  is  curved,  the  different  rays  of 
any  pencil  or  beam  will  be  reflected  differently,  because  the 
curvature  of  the  surface  makes  the  angle  at  which  the  ray 
is  reflected  different  from  what  it  would  be  if  the  surface 
was  plane,  and  thus  the  condition  of  the  rays  in  relation  to 
each  other  is  very  materially  altered  by  such  reflection. 
Parallel  rays  may  become  convergent  or  divergent,  and 
divergent  rays  may  become  convergent  or  parallel. 

It  is  the  same  in  respect  to  refraction.  If  the  surface  is 
plane,  so  that  all  the  rays  entering  it  are  subject  to  refrac- 
tion under  the  same  conditions,  they  are  all  bent  in  the 
same  manner;  and  if  they  at  last  enter  the  eve  and  form  an 
image,  the  image  is  not  changed  in  any  thing  except  in  ap- 
parent direction  from  which  the  rays  forming  it  come  to 
the  eye. 

This  is  what  happens  when  we  look  at  an  object  seen  ob- 
liquely under  water;  it  seems  raised  somewhat, but  is  not 


MATHEMATICAL   INVESTIGATIONS.  241 

altered  in  shape.  In  the  experiment  of  the  bent  pole,  for 
example,  the  part  which  is  beneath  the  water  seems  bent 
upward,  but  not  otherwise  altered — that  is,  the  rays  of 
light  being  all  refracted  in  coming  out  of  the  water,  with- 
out any  change  in  their  relative  positions,  but  only  in  the 
direction  in  which  the  whole  system  enters  the  eye,  the 
submerged  pole  is  altered  in  position  only,  and  not  in 
shape. 

Of  course,  as  there  is  absolutely  no  limit  to  the  forms  of 
curved  surfaces,  nor  to  the  changes  in  the  character  and 
condition  of  the  different  beams  and  pencils  of  light  falling 
upon  them,  the  phenomena  resulting  both  from  reflection 
and  refraction  are  infinite  in  number  and  variety.  To  un- 
derstand the  subject  fully,  in  all  its  possible  ramifications, 
must,  of  course,  transcend  the  power  of  the  human  mind; 
for  the  circle  of  phenomena  widens  and  expands  in  every 
direction,  and  the  facts  run  into  an  infinity  of  complicated 
details,  where,  of  course,  the  finite  powers  of  the  human 
mind  can  not  follow  them.  The  study  has,  however,  been 
carried  very  far  by  mathematical  opticians ;  and  the  inves- 
tigations which  they  have  made,  and  the  engravings  which 
have  been  executed  to  illustrate  the  results,  fill  volumes. 
It  would  not  be  possible,  in  such  a  work  as  this,  to  give 
even  a  summary  of  these  results. 

There  is,  however,  both  in  the  case  of  reflection  and  re- 
fraction, a  simple  general  principle  on  which  all  these  re- 
sults depend,  and  which  it  is  important  that  every  one 
should  have  in  mind.  This  principle  is  in  each  case  the 
key  to  all  the  phenomena,  however  complicated,  and  it  en- 
ables us,  if  we  possess  it,  to  understand  a  great  many  won- 
derful effects  taking  place  every  day  around  us  whicli 
would  be  otherwise  mysterious  and  unintelligible.  I  shall 
explain  these  two  principles  as  well  as  I  can,  first  in  com- 
mon language,  and  then  give  the  mathematical  expression 
L 


242  LAWS    OF   REFLECTION   AXD    REFRACTION. 

of  them,  which  we  shall  see  is  far  more  definite  and  pre« 
cise. 

In  the  case  of  reflection,  then,  the  light  rebounds,  as  it 
were,  in  a  manner  almost  precisely  analogous  to  the  re- 
bounding of  a  ball.  If  it  comes  to  any  surface  in  a  sloping 
direction  on  one  side,  it  goes  off  in  a  precisely  equally  slop- 
ing direction  on  the  other  side.  If  a  boy  standing  before 
a  wall  throws  his  ball  squarely  against  it,  it  comes  back 
squarely  toward  him.  If  he  throws  it  in  an  oblique  direc- 
tion upward,  so  that  it  strikes  the  wall  above  him,  it  re- 
bounds upward,  or  tends  so  to  rebound,  in  a  line  of  direc- 
tion pointing  as  much  above  as  the  line  of  direction  of  its 
approach  was  from  below.  It  is  true  that  the  weight  of 
the  ball — that  is,tue  influence  of  gravitation — immediately 
begins  to  turn  it  from  its  upward  course,  and  soon  brings 
it  back  to  the  ground.  In  the  actual  rebounding,  however, 
as  produced  by  the  simple  elasticity  of  the  ball,  the  obliq* 
uity  is  equal  on  each  side  of  the  point  on  which  the  ball 
impinges. 

It  is  the  same  with  light.  The  ray  moves  off  from  the 
point  in  the  mirror  where  it  strikes  in  a  direction  just  as 
far  on  one  side  as  it  came  to  it  on  the  other.  This  princi- 
ple has  already  been  stated,  and  to  some  extent  explained, 
in  the  chapter  on  "  Spectres  and  Ghosts,"  where  the  opera- 
tion of  it  was  to  be  specially  observed.  The  language, 
however,  in  which  we  have  here  stated  it  is  very  vague, 
and  does  not  give  the  law  with  any  degree  of  precision. 
The  mathematical  statement  is  much  more  fixed  and  de- 
terminate. The  ray,  in  coming  to  the  mirror,  is  called  the 
incident  ray /  in  leaving  the  mirror,  after  reflection,  it  be- 
comes the  reflected  ray.  The  obliquity  of  its  course  in 
coming  is  called  the  angle  of  incidence,  which  is  the  angle 
made  by  the  line  of  this  course  with  a  line  perpendicular 
to  the  reflecting  surface.  The  obliquity  of  its  course  ifi 


LAW    OF    REFLECTION.  243 

departing  is  called  the  angle  of  reflection.  It  is  the  angle 
formed  by  the  line  of  this  course  and  the  same  perpendic- 
ular. 

The  law,  then,  as  stated  mathematically,  is, 
That  the  angle  of  reflection  is  equal  to  the  angle  of  in- 
cidence, 

Which  is  only  a  more  definite  and  precise  way  of  say- 
ing that  the  ray  moves  off  from  the  point  in  the  mirror 
where  it  strikes  in  a  direction  just  as  far  on  one  side  as  it 
comes  to  it  on  the  other. 

The  diagram  illustrates  this  very  clearly.  S  S  is  the  re- 
flecting surface ;  I  o  the  course 
of  the  incident  ray,  and  R  o  the 
course  of  the  same  ray  after  re- 
flection. The  line  op  being  the 
perpendicular,  I  op  becomes  the 

g % 3  angle  of  incidence,  andpoH  the 

DIAGRAM.  LAW  OF  DEFLECTION,  angle  of  reflection.  The  law  is, 
that  in  all  cases,  and  whatever  may  be  the  direction  in 
which  the  ray  I  o  comes,  the  line  o  R,  into  which  it  is 
turned  by  reflection,  will  always  be  such  that  p  o  R  shall 
be  equal  to  p  o  I. 

If  this  law  is  once  understood  and  made  familiar,  you 
will  always  see  at  once  how  rays  will  be  reflected  from 
any  surface  the  form  and  character  of  which  you  know. 
If  the  ray  comes  to  the  surface  in  a  line  perpendicular  to 
it,  it  will  be  reflected  back  in  the  same  line.  If  it  comes 
on  either  side  of  the  perpendicular,  it  will  be  reflected  back 
with  the  same  degree  of  obliquity  on  the  other  side. 

The  action  which  takes  place  in  accordance  with  this 
law,  in  the  case  of  a  concave  mirror,  with  light  falling 
upon  it  in  one  particular  way,  is  shown  very  clearly  in  the 
engraving  on  the  following  page. 

The  point  o  being  the  centre  of  the  curvature  of  the 


244  LAWS    OF    REFLECTION    AND    REFRACTION. 


BEFLBOTION  FBOM  A  OONOAVK  BCBFAOE. 

mirror,  every  line  drawn  from  it  to  the  mirror  will  be  per- 
pendicular to  the  surface  at  the  point  where  it  meets  the 
surface.  Of  course,  rays  coming  from  any  point  to  the 
mirror  outside  those  lines  will  be  reflected  inside  them,  as 
is  seen  in  the  upper  ray  from  the  candle  B,  which,  after  re- 
flection, must  go  to  the  point  b,  making  the  angle  of  reflec- 
tion equal  to  the  angle  of  incidence.  If  rays,  instead  of 
coming  from  a  near  object  like  B,  come  from  a  more  dis- 
tant one,  the  angles  of  incidence  would  be  greater ;  and  if 
they  came  from  a  distance  so  great  as  to  make  them  sensi- 
bly parallel — as  from  the  sun,  for  example — then  the  lines 
of  incidence  would  be  farther  from  the  perpendicular  on 
the  outside,  and  those  of  reflection  would  be  farther  on  the 
inside,  so  that  the  rays  would  meet  in  a  point  nearer  the 
glass,  as  at  F,  which  is  called  the  focus  of  parallel  rays. 

This  example  shows  clearly  the  general  principle  on 
which  all  the  calculations  in  respect  to  the  effects  pro- 
duced by  reflection  are  founded.  Every  thing  depends 
upon  the  position  of  the  perpendicular  in  relation  to  the 
incident  ray,  or,  in  other  words,  the  angle  at  which  the 
incident  ray  comes  to  that  part  of  the  surface  by  which  it 
is  to  be  reflected.  Of  course,  as  the  directions  of  the  com- 
ing rays  and  the  forms  of  the  surfaces  may  be  infinitely 
raried,  the  special  effects  resulting  are  infinitely  varied 
too ;  but  this  one  simple  principle  governs  them  all. 


LAW    OF   REFRACTION.  245 

In  the  case  of  refraction — that  is,  the  modifying  of  the 
course  of  a  ray  of  light  in  passing  through  a  transparent 
substance,  instead  of  being  reflected  from  one  that  is 
opaque — the  law  is  in  itself  equally  simple,  though  it  is, 
perhaps,  not  quite  so  easily  stated.  The  best  way  for  you 
to  picture  it  to  your  minds  is  perhaps  to  consider  that 
when  a  ray  enters  water,  for  example,  obliquely,  it  has 
the  mass  of  the  water,  or  a  larger  portion  of  it,  in  closer 
proximity  to  it  on  one  side  than  on  the  other  at  the  in- 
stant of  entering  /  or,  as  it  might  perhaps  be  expressed, 
it  comes  sooner  in  contact,  or  more  fully  in  contact  with 
it  on  the  side  toward  which  it  inclines  than  on  the  other 
side. 

What  I  mean  by  this  is  shown  in  the  diagram,  where 
W 10  represents  the  water,  S  S 
the  surface  of  it,  and  I  o  the 
incident  ray.  Now,  whatev- 
er may  be  the  nature  of  the 
force  represented  by  the  ray 
"s  of  light,  and  whatever  maybe 
the  action  of  the  water  upon 
it  as  it  passes  into  the  water 
out  of  the  air,  it  is  easy  for  us 
to  imagine  that,  in  entering 
it  any  point,  as  at  o,  it  must 
come  sooner,  or  more  fully,  under  the  action  of  the  water 
which  is  on  the  side  W,  toward  which  it  is  inclined  at  the 
point  of  entering,  than  on  the  side  w,from  which  it  is  in- 
clined. Indeed,  in  former  times,  when  light  was  believed 
to  consist  'of  solid  particles  impelled  with  great  velocity 
through  the  air,  it  was  thought  that  the  bending  of  the  ray 
to  one  side  in  this  case  was  caused  by  the  more  powerful 
attractive  force  exerted  by  the  greater  mass  of  water  on 
that  side  at  the  instant  of  entering.  At  any  rate,  this  idea 


DIAGRAM.      LAW  OP  REFK ACTION.  ^V  aj 


246  LAWS    OF    REFLECTION   AND    REFRACTION. 

makes  it  very  easy  in  all  cases  to  see  in  what  direction  tlie 
ray  always  really  bends  in  passing  out  of  a  rare  medium 
into  a  denser  one. 

In  the  converse  case,  namely,  that  of  a  ray  passing  from 
a  dense  medium  into  a  rare  one,  the  effect  is  exactly  the 
converse — that  is,  a  ray  coming  in  the  line  R  o,  into  v>-  hich 
the  incident  ray  I  o  had  been  refracted,  will,  at  the  instant 
of  its  issuing  from  the  water,  o,  be  held  back,  as  it  were,  by 
the  superior  influence  over  it  of  the  greater  mass  of  water 
on  the  side  W  that  is  near  enough  to  act  upon  it  at  the 
moment  of  emerging,  than  by  that  on  the  side  w,  and  so 
will  be  drawn  down  into  the  direction  o  I,  which  is  precise- 
ly the  same  as  that  of  the  incident  ray.  Thus  the  action 
of  the  water  on  entering  and  departing  rays  is  equal  and 
reciprocal. 

This  mode  of  stating  the  case  is  very  indefinite  and 
vague,  and  would  be  wholly  unsatisfactory  considered  in  a 
scientific  point  of  view.  It  helps  us  very  much,  however, 
in  fixing  in  our  minds  the  general  law,  to  consider  that,  in 
passing  from  a  rare  to  a  dense  medium,  the  ray  is  bent  to 
ward  the  side  where  the  mass  of  the  dense  medium  lies 
nearest  to  the  course  it  is  following. 

Stated  scientifically,  however,  the  law  is,  that  the  ray,  in 
passing  from  a  rare  to  a  dense  medium,  is  bent  toward  the 
perpendicular  drawn  from  the  point  at  which  it  enters.  In 
passing  from  a  dense  to  a  rare  one,  it  is  bent/rom  the  per- 
pendicular to  precisely  the  same  extent. 

The  diagram  already  referred  to  shows  this  very  clearly. 
The  dotted  line pp  represents  the  perpendicular;  lo  is  the 
incident  ray,  entering  the  water  at  o.  Instead  of  going  on 
in  a  straight  course,  as  represented  by  the  dotted  line  o  v, 
it  is  bent  toward  the  perpendicular  into  the  line  o  R.  A 
ray  transmitted  in  the  contrary  direction — that  is,  from  R 
to  o,  instead  of  continuing,  when  it  emerges  from  the  water, 


GENERAL   PRINCIPLE.  247 

in  the  same  direction,  is  bent  away  from  the  perpendicular 
and  into  the  line  o  I. 

In  almost  all  cases  of  light  passing  from  one  transparent 
medium  into  another  of  a  different  internal  constitution, 
the  ray  is  bent  on  this  principle.  The  effect  in  most  cases 
seems  to  depend  upon  the  difference  of  density  iu  the  two 
media,  but  not  always.  There  is  some  mysterious  quality 
or  condition  of  structure,  not  well  understood,  on  which 
the  effect  depends.  The  substance  ordinarily  employed 
for  refracting  light  is  glass,  and  a  glass  formed  with  curved 
surfaces  to  produce  any  of  the  various  effects  which  may 
be  required  is  called  a  lens.  Lenses  are  of  various  kinds, 
according  to  the  effect  which  it  is  desired  to  produce.  A 
double  convex  lens,  for  example,  is  convex  on  both  sides. 
Its  effect  upon  parallel  rays,  according  to  the  principle  al- 
ready explained,  namely,  that  the  light  is  bent  toward  the 
side  where  the  largest  portion  of  the  substance  of  the  glass 
comes  nearest  to  it,  or,  in  other  words,  toward  the  perpen- 
dicular at  the  point  where  it  enters,  is  to  draw  the  rays 
all  inward,  as  we  see  is  the  case  with  a  sun-glass,  which  is 
an  example  of  a  double  convex  lens.  On  the  other  hand, 
the  same  principle,  in  the  case  of  a  double  concave  lens, 
which  is  the  kind  used  for  the  eye-glasses  of  near-sighted 
persons,  will  tend  to  spread  the  rays  instead  of  drawing 
them  together;  for  while,  in  the  convex  lens,  the  thickness 
of  the  glass  increases  toward  the  centre,  in  one  that  is  con- 
cave the  thickness  increases  from  the  centre  to  the  circum- 
ference, and  the  light  is  drawn  away  toward  the  side 
where  the  greatest  thickness  lies  at  the  point  at  which  it 
enters. 

It  is  easy,  in  the  same  manner,  for  one  who  has  the  prin- 
ciple of  reflection  in  mind,  as  it  has  been  explained  in  this 
chapter,  to  see  in  what  way  light  will  be  reflected  from  a 
polished  surface  in  any  specified  position  or  of  any  speci- 


248  LAWS    OF   REFLECTION   AND   REFRACTION. 

fied  curvature.  He  has  only  to  consider  in  what  way  solid 
bodies,  impelled  against  such  a  surface,  would  rebound  from 
it.  Thus  a  concave  mirror  acts  to  gather  together  parallel 
rays  falling  upon  it,  just  as  a  shower  of  peas  falling  upon 
the  inner  sides  of  a  saucer  or  a  bowl  would  rebound  to- 
ward the.centre.  On  the  other  hand,  a  convex  mirror  will 
reflect  rays  in  a  divergent  direction,  just  as  peas  falling  in 
a  shower  upon  the  outside  of  a  bowl,  placed  bottom  up- 
ward, would  rebound  away  from  it  in  every  direction. 


THE    BUTTERCUP    EXPERIMENT.  249 


CHAPTER  XXVH. 

THE    EYE. 

WHEN  a  child  holds  a  buttercup,  in  a  bright  sunny  day, 
under  the  chin  of  another  child,  if  the  light  happens  to 
come  right,  a  slight  yellow  tinge  appears  upon  the  skin 
opposite  to  it. 

There  are  two  explanations  of  this  phenomenon.  One 
is  the  notional,  and  the  other  the  scientific  one. 

The  notional  explanation,  which  is  the  one  generally 
adopted  by  children,  is,  that  the  child  on  whom  the  exper- 
iment is  performed  "  loves  butter." 

The  scientific  explanation  is,  that  the  petals  of  the  but- 
tercup having,  in  some  mysterious  way  which  no  one  pre- 
tends to  understand,  the  power  of  separating  the  rays  of 
white  light  which  fall  upon  them  from  the  sun  into  their 
component  parts,  and  of  absorbing  all  but  the  yellow  rays, 
these  yellow  rays  are  reflected  upward,  and,  falling  upon 
the  chin  at  a  place  somewhat  sheltered  from  the  bright 
light  of  the  sun,  are  reflected  to  the  eyes  of  the  children 
looking  on.  This  second  reflection  depends  altogether 
upon  the  brightness  of  the  light  shining  upon  the  butter- 
cup and  the  relative  position  of  the  surfaces  on  which  the 
light  shines,  and  not  at  all  on  the  taste  or  inclination  of 
the  subject  of  the  experiment  in  respect  to  butter. 

This  case  is  a  pretty  fair  illustration  of  the  difference  be- 
tween the  notional  and  the  scientifical  explanations  of  the 
phenomena  taking  place  in  nature  all  around  us  at  all 
times. 

The  yellow  rays,  as  we  call  them — though  we  must  not 
L2 


250  THE    EYE. 

forget  that  what  we  mean  by  this  term  is,  not  that  the 
rays  are  yellow  in  themselves,  but  only  that  they  have  the 
power  of  producing  that  sensation  in  us  when  they  enter 
our  eyes — undergo,  in  the  case  of  the  buttercup  and  the 
chin,  two  rejections :  one  from  the  petals  of  the  flower,  by 
which  they  are  separated  from  the  other  rays,  and  the  oth- 
er from  the  chin.  Any  color  may  be  thus  reflected  a  sec- 
ond time,  and  the  effect  is  more  or  less  distinct  accord- 
ing as  the  second  surface  is  more  or  less  shaded  from  all 
extraneous  light — that  is,  light  coming  to  it  from  other 
sources.  Thus  almost  any  object  held  near  a  red  curtain 
which  the  sun  shines  upon  will  look  red  by  reflected  light. 
In  this  case  the  red  light  from  the  curtain  constitutes  so 
large  a  portion  of  all  the  light  which  the  object  receives 
that  the  reflection  of  it  becomes  visible. 

In  the  same  manner,  all  the  objects  in  an  ordinary  room 
reflect  each  its  own  colored  rays  to  every  part  of  the  room. 
These  rays  mingle  and  blend  with  each  other  in  passing 
through  the  air,  and  if  we  hold  up  a  sheet  of  paper  as  a 
screen,  they  all  fall  upon  it  together,  in  combination  with 
the  white  light  of  the  sun,  so  that  the  paper  reflects  only 
a  mingled  and  general  light  to  our  eyes.  But  if  a  lens  is 
interposed  in  a  proper  manner  between  the  paper  and  any 
group  of  these  objects,  and  all  other  light  is  excluded,  then 
the  differently  colored  rays  from  all  these  objects,  and  from 
the  different  parts  of  the  same  object,  are  made  to  converge, 
and  are  brought  to  a  focus,  each  in  its  proper  place,  and  a 
distinct  image  of  the  whole  group  is  formed,  with  all  the 
parts  in  their  proper  position,  and  of  their  proper  color. 

How  this  is  effected  is  shown  very  clearly  in  the  image 
of  the  lily  in  the  engraving.  The  rays  from  only  two 
points  (A  and  B)  are  shown,  but  the  same  effect  takes 
place  in  respect  to  the  light  issuing  from  every  other  point 
in  the  flower.  All  these  rays,  in  passing  through  the  air, 


FORMATION    OP    AX    IMAGE. 


251 


MAGK   OF  THE    LILY. 


are  completely  intermingled,  though  each  one,  wonderful 
as  it  seems,  pursues  its  way  uninterrupted  and  undisturbed 
by  the  rest.  A  screen  held  at  the  place  of  the  lens  would 
receive  them  blended  together,  and  would  reflect  their 
united  light  oi>!y  as  a  general  illumination.  But  the  lens 
causes  each  separate  pencil,  coming  from  every  different 
point,  to  converge  each  toward  its  own  central  line.  The 
result  is  that  the  colors  are  all  separated;  and  if  the  screen 
is  held  iu  the  proper  place  to  receive  them,  and  all  light 
from  other  sources  is  excluded,  a  perfect  image  of  the  lily 
is  formed,  only  it  is  inverted,  since  the  several  pencils  cross 
each  other  at  o  in  traversing  the  lens;  those  from  A,  for 
example,  coming  to  a  focus  at  a,  and  those  from  B  at  b. 

This  experiment  can  be  easily  performed  by  means  of 
any  convex  lens,  such  as  a  reading-glass,  a  sun-glass,  or 
even  one  of  the  glasses  of  a  pair  of  spectacles  such  as  are 
used  by  elderly  persons.  Near-sighted  glasses,  being  con- 
cave instead  of  convex,  and  so  causing  the  rays  to  diverge 
instead  of  to  converge,  of  course  will  not  answer. 

The  only  difficulty  in  making  this  experiment  perfectly 
successful  is  that  of  keeping  all  other  light  except  that 
which 'comes  through  the.  lens  away  from  the  screen,  or 
from  whatever  serves  as  a  screen,  to  receive  the  image 


252 


THE    EYE. 


But  if  you  make  the  experiment  in  the  evening,  and  with 
only  one  lamp  in  the  room,  or,  when  there  are  more  than 
one,  if  they  are  placed  near  together,  and  thi'ow  the  image 
on  the  wall,  or  on  a  sheet  of  paper  held  near  the  wall,  an 
inverted  picture  of  the  flame  or  flames,  of  beautiful  dis- 
tinctness, will  be  formed. 

Images  of  any  other  objects,  as  well  as  of  bright  flames, 
can  be  produced  in  this  way,  if  only  all  extraneous  light 
can  be  excluded.  This  is  exactly  what  is  done  in  the  eye. 
The  eye  is  simply  a  space  inclosed,  with  an  opening  in  the 
front  part  of  it,  where  a  double  lens  is  placed  to  receive 
the  light,  all  other  light,  except  the  rays  that  come  through 
the  lens,  being  excluded.  An  image,  then,  of  any  outward 
object  toward  which  the  eye  is  directed,  is  formed  upon  a 
peculiar  membrane  at  the  back  of  it  called  the  retina, 
which  serves  as  a  screen.  Of  course  the  image  is  invert- 


EOTION   OF   TIIK   EYE. 


THE    CAMERA   OBSCUEA.  253 

ed.  How  it  happens  that  we  see  things  right  side  up 
when  the  picture  that  is  formed  in  the  eye  by  which  we 
see  them  is  upside  down,  is  a  mystery  which  greatly  puz- 
zles the  philosophers. 

If  an  exact  model  of  the  eye  were  made  of  porcelain  and 
glass,  with  a  little  peep-hole  upon  one  side,  so  that  we  could 
look  in,  we  should  see  in  the  interior  of  it,  on  the  back  side, 
a  most  perfect  and  beautiful  picture  of  any  external  scene 
or  object  toward  which  the  opening  in  front  might  be 
turned.  A  great  variety  of  optical  instruments  have  been 
invented  by  man  which  act  on  the  same  principle  as  the 
eye.  There  is  a  lens  to  concentrate  the  rays,  a  screen  to 
receive  the  image,  and  an  inclosure  to  exclude  all  other 
light  except  what  comes  through  the  lens.  There  is  also 
often  a  mirror  to  reflect  the  image,  so  that  the  screen  that 
receives  it  may  be  placed  where  it  may  be  most  conven- 
iently viewed. 

There  is  another  advantage  in  the  use  of  the  mirror  in 
these  cases,  for,  by  reflection  in  it,  the  image  may  be  thrown 
upon  a  horizontal  screen,  and  in  that  case  it  may  be  looked 
at  from  the  side  that  will  bring  it  right  side  up. 

There  are  many  ways  by  which  these  arrangements  are 
inclosed  for  the  purpose  of  excluding  the  outside  light;  for, 
in  order  to  produce  the  full  effect,  it  is  necessary  that  all 
light,  except  what  comes  from  the  objects  to  be  viewed, 
should  be  excluded.  Indeed,  these  instruments  all  take 
their  name  from  the  Latin  words  meaning  dark  chamber, 
or,  rather,  chamber  dark,  which  words  are  camera  obscura. 

The  following  engraving  shows  one  of  the  forms  in  which 
the  camera  obscura  is  often  made.  The  rays  R,  which  enter 
the  tube  B,  are  made  to  converge — that  is,  all  which  come 
from  any  one  point  in  the  object  are  made  to  converge, 
and  they  would  fall  upon  the  back  of  the  box  O,  and  form 
an  image  there,  were  it  not  that  they  are  reflected"  by  the 


254  THE    EYE. 


CAMERA    OBSC 


sloping  mirror  M  up  to  a  sheet  of  thin  paper  laid  upon  a 
glass  plate  above,  where  the  observer  can,  if  he  pleases, 
make  a  tracing  with  his  pencil  of  the  picture  they  produce. 

Sometimes  the  inclosure  to  exclude  light  from  the  sides 
consists  of  a  darkened  room.  The  apparatus,  however,  for 
forming  the  image  in  such  a  case  is  substantially  the  same, 
consisting  of  a  lens  to  form  the  image,  and  a  mirror  to  pro- 
ject it  where  it  is  most  convenient  to  place  the  screen. 
Sometimes  the  entering  rays  are  reflected  in  the  mirror 
first,  and  afterward  passed  through  the  lens.  It  is  neces- 
sary in  all  these  cases  that  the  room  should  be  darkened 
by  means  of  shutters,  or  in  some  other  way.  The  appara- 
tus is  usually  fitted  into  an  opening  made  in  one  of  the 
shutters,  while  the  others  are  entirely  closed. 

In  order  to  avoid  the  inconvenience  of  darkening  a  large 
room  in  this  way,  a  small  building,  like  a  summer-house,  is 
sometimes  devoted  exclusively  to  the  purpose  of  a  camera 
obscura ;  this  is  often  done  in  large  public  gardens  or 
pleasure-grounds. 


THE  ARTIST'S  TENT. 


257 


Sometimes  a  camera  obscura  is  fitted  up  upon  wheels, 
like  a  traveling  photographic  apparatus,  for  a  show ;  and 
sometimes  in  a  tent,  for  the  use  of  artists ;  only  in  this 
case  it  is  necessary  that  the  tent-cloth  should  be  perfectly 
opaque  and  dark. 


CAMERA   OB8CCBA   IN   A   TENT. 


The  tent  arrangement  is  attended  with  the  great  advan- 
tage that  it  can  be  removed  from  place  to  place,  and  can  be 
set  up  in  situations  inaccessible  to  a  wheeled  carriage. 
In  the  engraving  the  tent  is  open  in  front,  being  drawn 
so  in  order  to  allow  us  to  see  the  interior;  and  the  cloth 
does  not  quite  come  to  the  ground,  in  order  that  we  may 
see  the  supports.  In  actual  use  it  ought  to  be  closed  en- 
tirely, except  at  the  opening  in  the  apparatus  at  the  top  to 
admit  the  light  which  is  to  form  the  picture. 

The  process  of  photography  consists  simply  of  fixing  the 
image  produced  in  the  camera  obscura.  The  box  used  is 


258  THE    EYE. 

essentially  the  same  with  the  one  above  described,  but  the 
paper  on  which  the  image  is  finally  received,  when  all  is 
ready  and  the  picture  is  to  be  taken,  is  made  sensitive  by 
being  covered  with  a  chemical  substance  which  is  affected 
by  the  light. 

There  is  an  immense  number  of  optical  instruments, 
greatly  varied  in  their  construction  and  in  the  purposes 
which  they  serve,  which,  however,  all  depend  upon  the  op- 
eration of  the  simple  principles  of  reflection  and  refraction 
which  have  been  explained.  To  enter  into  a  description, 
or  even  an  enumeration  of  them,  would  be  foreign  to  the 
purpose  of  this  work,  which  is  simply  to  unfold  and  explain 
the  grand  fundamental  principles  that  are  exemplified  in 
the  action  of  light,  as  it  exhibits  itself  to  us  in  the  phenom- 
ena of  nature  around  us. 

There  is  one  principle  which  is,  however,  only  in  a  sec- 
ondary sense  a  property  of  light,  which  I  must  explain  be- 
fore closing  this  chapter,  and  that  on  account  of  the  great 
interest  which  John  and  Flippy  took  in  it,  and  in  making  a 
certain  class  of  toys  illustrative  of  it. 

The  principle  is  called  the  Persistence  of  Vision.  The 
phrase  denotes  a  certain  property  of  the  retina  of  the  eye, 
or  of  the  nervous  connection  between  the  retina  and  the 
mind,  or  rather,  perhaps,  a  property  of  light  in  relation  to 
these,  by  which  the  impression  made  upon  the  mind  does 
not  instantly  cease  when  the  image  is  made  to  vanish. 
Thus  a  succession  of  very  rapid  flashes  always  appear  to 
us  like  a  continued  light,  as  the  impression  left  by  one  does 
not  fade  before  another  comes  to  renew  it. 

A  great  many  ingenious  toys  are  constructed  on  this 
principle.  The  kind  which  principally  struck  Flippy's  fan- 
cy— chiefly,  I  suppose,  because  he  could  manufacture  them 
himself — consisted  in  making  two  different  pictures  on  the 
two  sides  of  a  card,  and  then,  by  attaching  strings  at  the 


THE   TWIRLING   CARD. 


261 


ends,  and  spinning  the  cards  rapidly  by  means  of  the 
strings,  the  impressions  of  the  two  pictures  would  be  com- 
bined in  the  eye,  on  account  of  the  image  produced  by  one 
not  fading  from  the  mind  before  the  other  came  to  join  it. 
One  of  the  pairs  of  pictures  which  the  boys  thus  made  con- 
sisted of  a  man  on  one  side  brandishing  a  stick,  and  on  the 
other  side  a  pig  running  away.  Thus,  when  the  card  was 
twirled,  you  saw  one  picture  consisting  of  a  man  driving  a 

Pig- 

The  boys  made  these  pictures  by  cutting  out  the  figures 
in  black  paper,  and  then  pasting  them  upon  the  cards.  The 
figures  were  not  very  well  shaped,  but  Flippy  said  that 
that  was  no  matter;  they  were  just  as  funny  for  all  that. 

Sometimes  they  drew  the  pictures  with  pen  and  ink,  and 
sometimes  they  painted  them  in  colors.  One  which  they 
drew  consisted  of  an  empty  cage  on  one  side,  and  a  bird, 
which  they  painted  of  a  bright  blue,  on  the  other.  When 
the  card  was  twirled  the  bird  was  seen  in  the  cage. 

The  scientific  name  for  this  contrivance  is  the  Thauma- 
trope. 


THE   THAUMATEOPE. 


£62  THE    EYE. 

They  also  painted  spots  of  different  colors  on  the  oppo- 
site sides  of  the  cards,  in  order  to  see  what  compound  color 
would  be  produced  in  blending  them  by  the  twirling  of  the 
card.  The  boys  spent  two  whole  days,  during  which  they 
were  confined  within  doors  by  rain,  in  making  a  number 
of  these  cards  of  various  styles  and  patterns.  They  made 
them  to  carry  home  with  them  to  America.  Lawrence 
highly  approved  of  this  amusement.  He  said  that  such 
cards  were  worthy  of  being  regarded  with  special  respect, 
in  view  of  their  being  capable  of  fulfilling  a  double  func- 
tion. They  were  equally  adapted  to  interest  children  as 
amusing  toys,  and  philosophers  as  articles  of  apparatus  il- 
lustrating the  persistence  of  vision. 


AEEIVAL   AT   LIVERPOOL.  263 


CHAPTER  XXVIII. 

THE    EETUKN. 

WHEN  at  length  the  time  arrived  for  Lawrence  and  John 
to  set  out  on  their  return  to  America,  John  had  learned  so 
much  about  the  philosophy  of  light,  both  from  the  books 
which  he  had  read  upon  the  subject,  and  from  the  conver- 
sations which  he  had  held  with  Lawrence,  and  he  had, 
moreover,  fixed  so  firmly  in  his  mind  what  he  had  learned 
by  the  notes  of  conversations,  and  the  other  articles  which 
he  had  written  in  his  book,  that  he  was  greatly  interested 
in  the  subject.  Indeed,  there  were  some  indications,  once 
or  twice,  as  Lawrence  observed,  of  his  beginning  to  feel  a 
little  vanity  and  self-conceit  in  view  of  the  progress  which 
he  had  made. 

After  traveling  slowly  and  by  a  somewhat  circuitous 
route  through  France  and  England,  Lawrence  and  John  ar- 
rived at  length  at  Liverpool,  a  day  or  two  before  the  sail- 
ing of  the  steamer  in  which  they  had  taken  passage  for 
America.  The  appointed  day  at  length  arrived,  and  they 
went  on  board  with  the  other  passengers,  and  the  steamer 
set  sail. 

It  was  late  in  the  fall  when  this  return  voyage  was  made, 
and  the  weather  for  several  days  was  stormy,  and  the  sea 
rough.  On  this  account,  and  also  because  this  was  the  re- 
turn voyage,  the  passengers  were  more  quiet,  and  kept  by 
themselves  more  than  on  the  voyage  out — that  is,  out  as 
the  Americans  call  it,  though  the  English  always  call  the 
voyage  to  America  the  outward  one,  and  that  from  Amer- 
ica to  England  the  voyage  home.  The  Americans  are  usu- 
ally much  more  quiet,  and  much  less  inclined  to  make  ac- 


264  THE    KETUKX. 

quaintance  with  each  other  on  the  voyage  back  to  Amer< 
ica,  at  the  end  of  the  tour,  than  when  crossing  the  ocean 
on  their  way  to  Europe,  at  the  beginning.  They  are  tired 
of  excitement  and  change,  and  their  minds  are  occupied 
with  recollections  of  the  scenes  they  have  visited,  the  pleas- 
ures they  have  enjoyed,  the  vexations  and  disappointments 
which  they  have  suffered,  and  with  thoughts  of  home. 

After  a  time  the  wind  and  the  waves  subsided,  and  Law- 
rence and  John  began  to  come  up  oftener  to  the  deck.  One 
day  when  they  were  sitting  there,  about  noon,  waiting  for 
the  officers  who  were  engaged  at  their  observations  to 
"make  it  12  o'clock,"  and  for  the  luncheon  bell,  which  they 
knew  would  be  rung  as  soon  as  the  waiters  should  hear 
eight  bells  strike,  John  took  out  his  watch,  and,  finding 
that  it  was  after  twelve  by  it,  he  asked  Lawrence  why  they 
did  not  strike  eight  bells. 
"It  is  after  twelve,"  said  he. 

"  But  you  have  not  got  our  true  time,"  said  Lawrence. 
"  Yes,"  replied  John,  "I  set  my  watch  by  the  ship's  clock 
this  morning." 

"Ah !  that  \vas  yesterday's  time,"  said  Lawrence.  "  We 
have  run  two  or  three  hundred  miles  to  the  westward  since 
yesterday,  arid  it  will  not  be  twelve  o'clock  here  until  the 
sun  has  had  time  to  come  all  that  distance  from  where  we 
were  when  it  was  noon  yesterday." 

John  then  asked  some  questions  about  the  mode  of  mak- 
ing observations  at  sea.  Lawrence  said  that  the  midday 
observation  was  for  the  latitude  only,  which  they  deter- 
mined by  the  altitude  of  the  sun  at  noon.  The  altitude  of 
the  sun,  when  it  passes  the  meridian,  varies  from  day  to 
day  for  the  same  place,  and  it  also  varies  with  the  distance 
of  the  place  from  the  equator ;  so  that  by  finding  the  alti- 
tude by  means  of  the  sextant,  and  looking  in  the  tables, 
the  particular  latitude  which  gives  that  altitude  on  that 


REFRACTION   BY   THE    ATMOSPHERE.  265 

day  is  readily  found.  There  are,  however,  several  correc- 
tions to  be  made. 

Lawrence  explained  some  of  these  corrections,  and  there 
was  one — namely,  that  for  refraction,  as  it  is  called — which 
John  was  much  interested  in,  because  his  studies  in  respect 
to  light  enabled  him  to  understand  it  very  clearly. 

It  seems  that  the  rays  of  light  from  the  sun,  in  passing 
from  the  inter-planetary  space  into  the  earth's  atmosphere, 
are  refracted,  and  thus  bent  downward  more  and  more  in 
passing  through  the  increasing  density  of  it.  This  effect 
is  greatest  when  the  sun  is  near  the  horizon,  and  it  makes 
the  sun  appear  higher  than  it  actually  is.  In  fact,  it  brings 
his  disk  into  view  before  he  has  really  risen. 


ATMOSPHERICAL   KEFRACT 


This  is  made  plain  by  the  engraving,  where  the  ray  of 
light  from  the  sun  (S),  while  it  is  below  the  line  of  the 
horizon  (H),  is  bent  downward  in  passing  through  the  suc- 
cessive portions  of  the  atmosphere  enveloping  the  earth, 
so  as  to  come  to  the  eye  of  the  spectator  at  A  as  if  it  real- 
ly proceeded  from  a  higher  point,  and  thus  brings  the  sun 
into  view  while  it  is  actually  below  the  horizon. 
M 


266  THE    KETURN. 

The  effect  is  greater  when  the  sun  is  low,  and  continual- 
ly diminishes  with  its  increasing  altitude;  but  the  navi- 
gators' books  contain  a  table  in  which  they  can  find  the 
proper  correction  to  be  made  in  every  case. 

John  was  quite  pleased  to  find  that  he  could  understand 
this  explanation,  and  the  drawing  which  Lawrence  made 
to  represent  it,  so  easily,  and  said,  after  a  moment's  pause, 
that  he  thought  that  he  had  learned  a  good  deal  about 
light  since  he  had  been  on  that  tour. 

"  Yes,"  said  Lawrence,  "  you  have  indeed.  You  have 
made  an  excellent  beginning.  But  the  field  of  knowledge 
widens  more  and  more  the  farther  we  advance  into  it. 
You  have  learned  a  great  many  of  the  first  principles,  and 
these,  being  fundamental,  are  of  great  importance.  But 
when  you  go  farther,  and  study  the  construction  and  phi- 
losophy of  the  microscope,  the  telescope,  the  magic  lan- 
tern, the  stereoscope,  and  the  analysis  by  the  spectroscope 
of  the  chemical  composition  of  incandescent  substances, 
however  remote  from  us,  you  will  think  that  what  you 
have  yet  learned  is,  after  all,  very  little.  And  when  you 
come  to  investigate  the  phenomena  of  diffraction,  and  in- 
terference, and  polarization,  you  will  almost  conclude  that 
you  know  now  nothing  at  all." 

"What  are  all  those  things,  any  how  ?"  asked  John. 

"  What  they  call  diffraction"  said  Lawrence,  "  is  a 
change  produced  in  some  mysterious  -way  in  the  move- 
ment of  rays  of  light  when  a  very  slender  beam  passes 
through  a  very  narrow  slit  or  opening,  or  by  the  side  of  a 
very  narrow  obstruction,  so  as  to  produce  fringes  of  differ- 
ent colors.  You  can  see  these  fringes  sometimes,  though 
very  irregularly,  when  you  look  at  a  bright  light  with  your 
eyes  almost  shut,  so  as  to  see  it  through  your  eyelashes. 
There  are  ways  of  producing  them  very  regularly  and 
beautifully  on  a  screen  by  means  of  suitable  apparatus, 


POLARIZATION. — INTERFERENCE.  267 

but  to  understand  clearly  how  they  are  produced  requires 
a  great  deal  of  mathematical  knowledge.  It  is  in  some 
way  by  which  the  pulsations  or  vibrations  of  different  rays 
mingle  or  combine  their  actions  so  as  to  produce  new  and 
strange  effects.  Sometimes  two  rays  entirely  neutralize 
each  other,  so  that  two  lights  make  darkness.  This  is 
what  they  call  interference. 

"As  to  polarization"  continued  Lawrence,  "that  is  more 
difficult  still  to  understand.  It  forms  quite  a  science  by  it- 
self, and  one,  too,  of  a  highly  mathematical  character.  Po- 
larized light  is  light  which  has  been  changed  in  a  certain 
way,  so  that  it  acts  differently  from  light  in  its  ordinary 
state,  and  produces  certain  beautiful  and  very  wonderful 
effects  in  the  microscope,  and  reveals  in  a  marvelous  man- 
ner certain  differences  in  the  internal  constitution  of  dif- 
ferent transparent  substances  which  could  be  discovered 
in  no  other  way. 

"  What  they  call  interference,"  continued  Lawrence,  "  is, 
as  I  have  already  said,  a  kind  of  combination  of  the  waves, 
by  which  the  swell  of  one  is  made  to  correspond  with  the 
hollow  of  another,  as  it  were,  and  so  they  are  both  extin- 
guished. Imagine  that  you  throw  a  stone  into  a  pond  and 
set  in  motion  circles  of  waves,  and  then  suppose  that  an- 
other stone  is  thrown  in  so  as  to  strike  at  precisely  the 
right  instant  to  make  a  second  set  of  waves  that  shall  ex- 
actly coincide  with  the  first  set.  This  would  tend  to  in- 
crease the  height  of  them." 

"Yes,"  said  John,  "I  admit  that." 

"But  now,"  continued  Lawrence,  "suppose  the  stone 
were  thrown  in  at  the  right  instant  to  make  the  hollows 
of  the  second  set  coincide  with  the  swellings  of  the  first. 
The  two  sets  of  impulses  would  then  neutralize  each  other 
— or  interfere,  as  they  call  it  when  speaking  of  light,  and 
the  water  would  remain  level." 


268  THE    KETUKN. 

"  It  could  not  be  done,"  said  John. 

"  True,"  said  Lawrence ;  "  but  can  it  not  be  imagined  ?" 

"  I  don't  think  I  can  hardly  imagine  it,"  said  John. 

"  Not  even  as  an  illustration  ?"  said  Lawrence. 

"I  don't  know,"  said  John,  speaking  doubtfully. 

"  You'll  have  to  imagine  it,"  said  Lawrence, "  if  you  wish 
to  get  an  idea  of  what  is  meant  by  interference  in  the  case 
of  light.  Besides,  though  you  say  it  would  be  impossible, 
perhaps,  to  do  this  with  waves  of  water,  the  effect  can  be 
produced  exactly  by  a  mechanical  apparatus  to  make  arti- 
ficial representations  of  waves." 

"  I  should  like  to  see  that  apparatus,"  said  John. 

"At  any  rate,"  continued  Lawrence,  "it  is  found  that 
rays  of  light,  or  luminous  impulses  following  each  other  in 
a  certain  way,  do  extinguish  each  other.  The  experiments 
are  very  complicated  and  very  curious,  but  they  are  thought 
to  prove  positively  that  light  really  consists  of  a  rapid  suc- 
cession of  some  kind  of  undulations  or  waves." 

"  Do  you  think  they  do  really  prove  that  ?"  asked  John. 

"  I  think  they  prove  the  existence  of  some  kind  of  inter- 
mittent action,  with  alternating  conditions  capable  of  in- 
tensifying or  neutralizing  each  other,  according  as  they 
agree  or  disagree ;  but  whether  the  successive  impulses 
are  of  the  character  of  vibrations  or  undulations  in  a  sub- 
tle ether,  I  do  not  know." 

Lawrence  was  right,  perhaps,  in  saying  that  he  was  not 
entirely  satisfied  in  respect  to  the  precise  nature  of  this 
mysterious  action ;  but,  at  any  rate,  it  seems  to  be  proved 
that  there  is  an  excessively  rapid  intermittent  force  of 
some  kind  or  other  that  is  concerned  in  the  production  of 
light,  and  the  length  of  the  several  pulses,  and  the  number 
which  are  produced  in  a  second,  seem  to  have  been  quite 
exactly  ascertained,  on  the  principle  of  determining  the 
interval  in  time  and  distance  which  is  requisite  to  produce 


BLOWING    BUBBLES. 


269 


the  nterference.  The  effects  of  this  interference  are  mani- 
fested in  many  very  remarkable  and  very  curious  phenom.' 
ena.  John  said  that  he  should  like  to  see  some  of  them. 
Lawrence  replied  that  it  was  easy  to  see  them,  but  not  so 
easy  to  understand  how  they  were  produced. 

"They  appear  in  various  colored  fringes,"  said  Law- 
rence, "  in  almost  all  transparent  substances  which  are 
made  extremely  thin — so  thin  that  the  light,  in  being  re- 
flected back  and  forth  from  one  surface  to  the  other,  is 
caused  to  '  interfere.'  We  can  make  a  thin  film  of  air 
which  will  show  them  by  pressing  two  plates  of  glass  to- 
gether which  have  surfaces  that  are  not  exactly  parallel. 
We  see  them  in  very  thin  plates  of  mica,  and  in  a  thin  film 
of  oil  or  other  such  substance,  floating  upon  water;  and, 


[SLOWING    IIUJtm.K8. 


270  THE   RETURN. 

better  still,  children  observe  and  admire  them  in  the  soap 
bubbles  which  they  blow.  The  colors  come  out  when  the 
bubble  grows  so  large  that  the  water  inclosing  it  becomes 
extremely  thin. 

"I  have  seen  the  colors  in  the  bubbles  very  often,"  said 
John, "  but  I  don't  understand  how  they  can  be  produced 
by  any  kind  of  interference  of  waves." 

"  No,"  replied  Lawrence, "  I  do  not  wonder  that  you  do 
not.  It  requires  a  very  profound  mathematical  study  to 
understand  it.  Newton  studied  it  in  that  way — " 

"  What !  with  a  soap-bubble  ?"  asked  John. 

"  Yes,"  replied  Lawrence ;  "  but  the  colors  moved  about 
so  much  when  the  bubble  was  floating  in  the  open  air,  and 
the  water  dried  from  the  surface  so  as  to  cause  it  to  burst 
so  soon,  that  at  first  he  met  with  a  good  deal  of  difficulty. 
He  saw  that  it  was  necessary  to  contrive  some  way  to 
remedy  these  evils,  so  he  blew  his  bubble  in  a  glass  globe, 
with  very  transparent  sides,  which  served  to  protect  it 
from  the  air,  and  which  he  previously  filled  with  moist  air 
in  order  to  prevent  the  evaporation." 

He  found  that,  when  thus  covered,  the  bubble  was  much 
more  permanent  than  when  exposed  in  the  open  air,  and 
the  colors  arranged  themselves  in  the  most  symmetrical 
and  beautiful  manner. 

"  I  mean  to  try  it  when  I  get  home,"  said  John. 

"I  would  do  so,  if  I  were  you,"  replied  Lawrence. 

"  Only,"  said  John, "  I  don't  know  how  I  can  get  a  glass 
globe." 

"  Any  kind  of  bottle  or  jar  would  do,  I  suppose,"  said 
Lawrence ;  "  only  you  must  have  a  stopper,  and  pass  the 
tube  that  you  blow  your  bubble  with  through  it,  so  as  to 
keep  the  moisture  all  in  the  jar,  in  order  to  prevent  the 
water  of  the  bubble  from  evaporating.  You  must  also 
stop  the  opening  into  the  pipe,  for  there  is  a  certain  con- 


THE    BUBBLE    PROTECTED. 


271 


fTOJS  8   HUBBLE. 


tractile  force  in  a  babble  which  gradually  begins  to  drive 
the  air  out  of  it  when  you  stop  blowing  in,  if  you  leave  the 
pipe  open.  You  see  this  by  the  bubble's  growing  gradu- 
ally smaller  and  smaller." 

When  Lawrence  and  John  had  arrived  at  about  this 
point  in  their  conversation,  the  officer  in  charge  struck 
eight  bells.  Those  who  had  been  making  observations  im- 
mediately went  below  with  their  sextants  and  the  lunch- 
eon-bell ran^. 


272  THE    RETURN. 

The  voyage  went  on  very  smoothly  and  pleasantly  aftei 
this,  though  every  one  seemed  more  than  usually  impa- 
tient to  reach  the  land. 

At  length,  just  before  the  time  arrived  for  the  land  to 
come  in  sight,  a  pilot-boat  appeared.  The  passengers  were 
all  very  much  interested  in  the  coming  of  the  pilot,  for 
they  expected  that  he  would  bring  them  the  news  which 
had  been  passing  under  the  Atlantic  from  Europe  to  Amer- 
ica, on  the  telegraph  wire,  since  they  left  Liverpool ;  and  as 
this  was  the  year  of  the  great  French  and  German  war, 
they  were  very  anxious  to  learn  what  had  happened  since 
they  left  the  English  shores.  When  the  pilot  came,  how- 
ever, they  were  much  disappointed  at  learning  that  his 
boat  left  New  York  only  the  day  after  the  steamer  had 
left  Liverpool,  so  that  he  could  give  the  passengers  only 
one  day's  later  news. 

It  was  a  joyful  hour  for  all  the  passengers  when  the 
steamer  was  sailing  up  the  harbor.  Home  seemed  to  them 
more  attractive,  after  all,  than  any  of  the  scenes  of  novelty 
and  beauty  which  had  enticed  them  abroad. 

The  immense  steamer  came  up  very  slowly  and  with 
much  difficulty  to  the  pier.  There  were  many  lines  taken 
out  in  boats  to  the  pier  and  fastened  there,  and  hard  pull- 
ing upon  them  by  the  sailors  at  windlasses  and  capstans, 
and  much  alternate  stopping,  and  backing,  and  going  for- 
ward of  the  engine.  There  were  crowds  of  people  all  this 
time  upon  the  pier  waving  hats  and  handkerchiefs,  which 
salutations  were  responded  to  by  the  passengers  on  board, 
who  crowded  the  promenade  deck  and  leaned  over  the 
railings  at  every  point  where  they  could  see. 

At  length  the  bow  of  the  steamer  was  brought  up  in  an 
awkward  position  among  the  piles  at  the  head  of  the  pier, 
and  a  broad  plank  platform  was  laid  across  for  the  transfer 
of  the  baggage  on  shore.  There  were  no  facilities  yet  for 


FLIPPY   AGAIN.  273 

the  passengers  to  land,  or  for  any  of  their  friends  to  come 
•,on  board.  A  few  adventurous  gentlemen,  however,  more 
'bold  or  more  agile  than  the  rest,  were  soon  seen  clamber- 
ing up  over  the  piles  and  getting  from  them  into  the  rig- 
ging, so  as  to  come  on  board.  Among  them  John's  eye 
fell  upon  a  boy  who  was  stopping  one  of  these  gentlemen 
and  asking  him  to  give  him  his  hand  to  help  him  across  a 
very  dangerous  place. 

"  Look !  Lawrence,  look !"  said  John ;  "  there's  Flippy ! 
I  verily  believe  that's  Flippy !" 

It  was  indeed  Flippy.  He  had  seen  in  the  newspaper 
the  names  of  Lawrence  and  John  in  the  list  of  the  passen- 
gers that  were  to  cross  the  Atlantic  in  that  steamer,  which 
had  been  telegraphed  to  New  York,  and,  being  in  New 
York  at  the  time,  had  come  down  to  welcome  them  to 
their  native  land. 

M2 


274  FAREWELL  TO   FLIPPY. 


CHAPTER  XXIX. 

FAREWELL   TO    FLIPPY. 

A  FEW  days  after  the  return  of  Lawrence  and  John  to 
New  York,  they  went  on  board  a  North  River  steam-boat 
to  go  up  the  river  on  their  way  to  their  home  in  the  coun- 
try. 

It  was  late  in  the  afternoon  when  they  went  on  board. 
On  their  way  from  the  hotel  to  the  pier,  John  said  to  Law- 
rence, in  the  carriage, 

"  It  would  have  been  better  for  us  to  have  planned  to 
go  up  in  the  day-boat." 

"  Why  so  ?"  asked  Lawrence. 

"  Because  then  we  could  have  seen  the  scenery  better," 
said  John. 

"That  is  not  a  settled  question,"  replied  Lawrence. 
"  Some  people  think  that  the  scenery  in  the  evening,  by 
starlight  or  moonlight,  is  a  great  deal  more  grand  and 
sublime,  especially  in  passing  through  the  Highlands." 

"  I  don't  care  much  about  that,"  said  John.  "  I  like  to 
see  them  stop  at  the  landings,  and  watch  the  people  going 
off  and  the  others  coming  on  in  the  day-time,  when  I  can 
see  them  plainly." 

"Yes,"  rejoined  Lawrence,  "I  should  expect  that  you 
would  take  more  interest  in  such  scenes  now  than  in 
mountains  by  moonlight.  You  have  not  yet  attained  to 
the  romantic  age." 

"  The  romantic  age  ?"  repeated  John. 

"  Yes,"  said  Lawrence.  "  I  divide  the  period  of  child- 
hood and  youth  into  four  ages.  First  comes  the  Wonder- 


ON    BOARD.  275 

ing  Age,  then  the  Noisy  Age,  then  the  Teasing  Age,  and, 
last  of  all,  the  Romantic  Age.  The  Romantic  Age  has  not 
come  for  you  yet." 

At  this  point  in  the  conversation  the  carriage  stopped. 
They  had  arrived,  it  seemed,  at  the  pier.  So  they  descend- 
ed from  the  carnage,  and,  after  paying  the  fare  and  attend- 
ing to  their  baggage,  they  went  on  board. 

"  Lead  the  way,  John,"  said  Lawrence,  as  they  stepped 
from  the  gang-plank  to  the  deck, "  and  find  the  place  where 
you  would  like  to  sit.  We  have  more  than  half  an  hour 
yet  before  the  steamer  will  start." 

John  replied  that  he  would  like  to  sit  where  he  could 
see  the  people  come  on  board ;  and,  so  saying,  he  led  the 
way  up  to  the  after  promenade  deck,  and  there,  choosing 
two  comfortable  arm-chairs,  he  brought  them  to  the  side 
of  the  deck  next  the  pier,  where  he  could  see  the  carriages 
and  carts  as  they  arrived,  and  the  foot  people,  and  the 
orange-women,  and  the  news-boys,  and  witness  at  his  ease 
all  the  exciting  scenes  and  incidents  which  usually  attend 
the  sailing  of  a  North  River  steamer  from  a  New  York  pier. 
As  soon  as  he  and  Lawrence  were  comfortably  established 
in  their  seats,  he  asked  Lawrence  to  go  on  with  what  he 
was  saying  about  the  ages  of  childhood  and  youth. 

So  Lawrence  went  on  to  explain  what  he  meant  by  the 
various  ages  that  he  had  specified.  The  Wondering  Age, 
he  said,  continued  from  infancy  till  the  boy  was  seven  or 
eight  years  old.  Up  to  that  time  the  world  was  all  new 
to  him,  and  his  mind  was  chiefly  occupied  with  curiosity 
and  wonder.  He  went  about  prying  into  every  thing. 
He  believed  every  thing  that  he  heard,  so  that  it  was  very 
easy  to  make  a  fool  of  him.  He  liked  fairy  tales,  and  the 
more  absurd  and  impossible  they  were,  the  better  he  was 
pleased  with  them. 

"  Next  comes  the  period  from  seven  or  eight  to  ten  or 


276  FAREWELL    TO    FLIPPY. 

twelve,"  continued  Lawrence,  "which  I  call  the  Noisy  Age. 
The  boy  has  by  this  time  become  somewhat  accustomed 
to  the  strange  world  that  he  finds  himself  brought  into, 
and  feels  more  at  home  in  it,  and  begins  to  see  more  clear- 
ly the  difference  between  truth  and  falsehood  in  it.  His 
powers  and  faculties  have  become  enlarged  and  developed, 
his  strength  is  increased,  and  he  begins  to  like  to  produce 
sensations  and  effects.  One  of  the  easiest  effects  that  he 
can  produce  is  noise.  He  likes  to  hear  it,  and  he  makes  a 
great  deal  of  it.  Indeed,  the  more  bustle  and  noise  there 
is,  the  better,  especially  if  he  makes  it  himself.  So  I  call 
this  the  Noisy  Age.  In  this  age  the  boy,  if  left  to  him- 
self, and  is  strong  and  healthy,  breaks  into  a  room  rudely 
where  people  are  quietly  talking,  and  if  reproved  and  asked 
to  be  more  quiet,  he  goes  out  sometimes  slamming  the  door, 
and  making  more  noise  in  going  out  than  he  did  in  coming 
in." 

"Yes,"  said  John,  smiling,  and  at  the  same  time  looking 
a  little  ashamed,  "  I  used  to  do  so." 

"  In  this  age,  too,"  continued  Lawrence, "  boys  are  fond 
of  rough  and  noisy  plays.  They  are  always  pushing  each 
other,  chasing  each  other,  and  tripping  each  other  up,  with 
a  vast  amount  of  shouting  and  hallooing  by  way  of  music- 
al accompaniment. 

"Next  comes  the  Teasing  Age,"  continued  Lawrence. 
"The  boy's  mental  faculties  have  now  become  somewhat 
more  fully  developed,  and  the  effects  that  he  now  likes  to 
produce  are  such  as  relate  somewhat  more  to  the  minds 
of  people  than  merely  to  their  eyes  and  ears.  He  takes 
special  pleasure  in  making  fools  of  people,  in  getting  boys 
or  dumb  animals  angry  with  each  other,  and  seeing  them 
fight.  If  he  has  any  sisters,  he  seems  sometimes  to  take 
special  pleasure  in  teasing  them.  That  is  the  reason  why 
I  call  it  the  Teasing  Age." 


THE   DIFFERENT   AGES. 


277 


EOUGII  PLAYS. 


"  I  think  you  ought  to  call  it  the  Ugly  Age,"  said  John. 

"  No,"  rejoined  Lawrence,  "it  is  not  exactly  from  a  spirit 
of  ugliness  that  he  does  these  things,  but  only  from  the 
pleasure  of  exercising  his  growing  powers  in  new  forms. 
To  produce  a  disturbance  or  an  excitement  in  a  person's 
mind  involves  the  exercise  of  a  higher  class  of  faculties 
than  merely  to  make  a  din  in  their  ears,  and  the  boy  likes 
to  exercise  his  highest  powers.  The  Teasing  Age  comes 
generally  between  ten  or  twelve  and  sixteen.  After  six- 
teen the  boy  generally  becomes  too  much  of  a  gentleman 
to  take  pleasure  in  troubling  people  in  any  way,  especially 
his  sisters.  He  then  becomes  ambitious  of  making  himself 
agreeable  instead  of  disagreeable." 

"J'ra  between  ten  and  sixteen,"  said  John, "  so  I  am  in 
the  Teasing  Age." 


278  FAREWELL  TO   FLIPPY. 

"Your  case  is  an  exception  to  the  general  rule,"  said 
Lawrence. 

"  I  think  there  are  a  great  many  exceptions,"  said  John. 

"  I  think  so  too,"  replied  Lawrence ;  "  indeed,  it  would 
not  be  a  good  specimen  of  a  general  rule  if  there  were  not 
a  great  many  exceptions." 

"  Look !  look !  Lawrence,"  said  John,  suddenly  interrupt- 
ing and  pointing  toward  the  pier ;  "  there  comes  Flippy  !" 

It  was  indeed  Flippy.  He  was  coming  down  the  pier 
with  a  parcel  in  his  hand.  John  ran  to  meet  him. 

In  a  few  minutes  he  returned,  bringing  Flippy  to  the 
place  where  Lawrence  was  sitting.  Flippy  placed  his  par- 
cel in  Lawrence's  hands,  saying  at  the  same  time, 

"  There  is  something  for  you ;  but  you  must  not  open  it 
until  you  get  home." 

"Is  it  a  present  for  me  from  you  ?"  asked  Lawrence. 

"Yes,"  replied  Flippy;  "though  my  father  gave  me  the 
money  to  buy  it,  because  you  were  so  kind  to  me  and 
taught  me  so  much  while  we  were  on  the  voyage.  And  I 
want  to  go  home  with  you  now,"  he  continued, "  to  where 
you  live." 

"  Oh  no  !"  rejoined  Lawrence ;  "  it  is  too  far ;  it  is  more 
than  two  hundred  miles  from  here." 

"  No  matter  for  that,"  said  Flippy ;  "  I  can  write  back  to 
my  father  at  the  first  place  where  we  stop,  and  he  will 
send  me  some  money.  He  won't  care,  so  long  as  he  knows 
that  I  am  witli  you." 

"  But  your  mother  would  care,"  replied  Lawrence ;  "  she 
would  be  very  anxious  and  very  much  worried  about  you." 

"  No  matter,"  said  Flippy.  "  She  would  find  out  after  a 
while  that  I  was  all  right." 

Lawrence  replied  that,  though  his  mother  might  find  out 
that  he  was  all  right,  as  he  called  it,  in  the  end,  she  would 
endure  a  great  deal  of  suffering  in  the  mean  time  through 


THE    BONNE.  279 

her  suspense  and  anxiety ;  and  then,  in  order  to  see  if  he 
could  not  awaken  some  sentiment  of  gratitude  in  his  mind 
toward  his  mother,  he  reminded  him  of  his  obligations  to 
her  for  all  the  care  and  trouble  which  she  had  borne  for 
him  in  former  years,  when  he  was  a  little  child ;  how  she 
had  attended  him  and  watched  over  him  in  sickness,  and 
sat  by  his  bedside  at  night,  and  provided  for  all  his  wants. 

"  My  mother  never  did  any  of  those  things  for  me,"  said 
Flippy. 

"  Who  did  them,  then  ?"  asked  Lawrence. 

"  Bonney,"  replied  the  boy. 

"And  who  was  Bonney?"  asked  Lawrence. 

"She  was  a  girl,  or  perhaps  a  woman,"  said  Flippy. 
"My  mother  called  her  the  bonne,  but  I  generally  called 
her  Bonney  —  generally,  but  not  always,  for  sometimes 
when  she  scolded  me  I  used  to  call  her  Bony." 

"  Did  she  use  to  scold  you  ?"  asked  Lawrence. 

"  Sometimes,"  said  Flippy,  "  especially  when  she  caught 
me  sliding  down  the  banisters." 


FLIPPY   WHEN  HE  WAS  LITTLE. 


"It  seems  to  me  it  was  hardly  right  to  call  her  a  bad 


280  FAREWELL   TO   FLIPPY. 

name,"  said  Lawrence, "  because  she  wished  to  prevent  you 
from  sliding  down  the  banisters.  It  was  only  out  of  re- 
gard for  your  safety  that  she  did  it.  I  knew  a  boy  once 
who  fell  and  broke  his  leg  by  sliding  down  the  banisters." 

"I  know,"  said  Flippy;  "but  I  could  poise  myself  ex- 
actly; besides,  it  was  not  a  very  bad  name  for  her,  for  she 
was  really  rather  bony." 

Just  at  this  moment  the  bell  rang,  and  a  steward  called 
out,  "All  ashore  that's  going  !"  So  Flippy  rose,  and,  bid- 
ding Lawrence  good-by,  he  and  John  went  down  the  com- 
panion-way to  the  main  deck,  and  there  Flippy  fell  into 
the  current  of  people  that  were  pouring  in  a  continued 
stream  over  the  plank  to  the  pier.  The  last  thing  that 
Flippy  said  was  that  he  wished  Lawrence  had  allowed  him 
to  go  with  him  and  John. 

"  I  might  have  gone  just  as  well  as  not,"  said  he, "  and  I 
could  have  written  to  my  father  at  the  first  stopping-place 
to  send  me  some  money  and  a  trunk  full  of  clothes." 

Before  John  had  made  his  way  back  to  where  Lawrence 
was  sitting,  the  steam-boat  had  begun  to  move  away  from 
the  pier,  and  very  soon  began  to  glide  very  swiftly  past 
the  long  line  of  ships,  and  ferry-boats,  and  canal-boats,  and 
sloops  which  lay  at  the  wharves  and  filled  the  docks  which 
here  formed  the  margin  of  the  river. 

"  I  like  Flippy  pretty  well,"  said  John,  as  soon  as  he  had 
resumed  his  seat, "  but  I  don't  think  he  is  very  grateful  to 
his  mother." 

"  It  is  partly  because  he  does  not  know  how  much  she 
has  done  and  suffered  for  him,"  replied  Lawrence.  "  There 
seems  to  be  a  principle  of  gratitude  in  his  heart,  or  else  he 
would  not  have  thought  of  bringing  a  present  to  me,  on 
account,  as  he  says,  of  my  having  been  kind  to  him." 

Here  Lawrence  held  up  the  parcel  which  Flippy  had 
given  him,  and  which  was  still  lying  in  his  lap. 


THE   PARCEL.  281 

"  What  I  have  done  for  him,"  he  added, "  little  as  it  is, 
he  knows  and  appreciates,  and  so  he  is  grateful  for  it.  But 
his  mother  has  perhaps  not  done  much  to  win  his  affec- 
tions of  late  years.  It  is  very  likely  that,  since  he  was  old 
enough  to  be  put  under  the  charge  of  a  bonne,  she  has  not 
had  much  to  do  with  him  except  to  watch  him  and  check 
him  when  he  is  doing  any  thing  wrong,  and  he  has  not  the 
least  idea  how  much  she  must  have  done  and  suffered  for 
him  before  that  time.  What  he  wants  is  light.  When  lie 
grows  older,  and  understands  how  much  he  owes  his  moth- 
er, it  is  very  probable  that  he  will  be  grateful  for  it  all,  and 
he  may  then  become  a  great  comfort  to  her.  I  am  sure  I 
hope  he  will." 

"  I  wonder  what  the  present  is  that  he  has  brought  for 
you !"  said  John.  "Let's  open  it  now." 

"  No,"  replied  Lawrence ;  "  I  was  not  to  open  it  till  we 
got  home." 

Here  John  took  lip  the  parcel  and  began  to  feel  of  it,  in 
hopes  of  being  able  to  ascertain  in  that  way  what  it  was. 

"I  thought  it  was  books,"  said  he, "but  it  is  some  kind 
of  box — a  pasteboard  box.  I  wonder  what  is  in  it !  If  I 
were  you,  I  would  open  it  now  and  see." 

"  I  was  not  to  open  it  until  we  got  home,"  said  Law- 
rence. 

"  You  did  not  promise  him  that  you  would  not,"  replied 
John. 

"  No,"  rejoined  Lawrence, "  I  did  not  promise  in  words, 
but  I  received  the  package  on  that  implied  condition." 

"  He  would  not  care,"  said  John.  "  All  he  wanted  was 
that  you  should  not  open  it  while  he  was  by.  I  don't  see 
what  possible  harm  it  could  do  for  you  to  open  it  now." 

"  I  do,"  said  Lawrence. 

"  What  harm  ?"  asked  John. 

"  Guess,"  said  Lawrence. 


282  FAREWELL   TO    FLIPPY. 

"That  it  might  be  some  delicate  thing  that  would  get 
injured  by  being  opened  here?"  suggested  John,  speaking 
in  the  tone  of  a  question. 

"That  is  a  pretty  good  reason,"  said  Lawrence, " but 
that  is  not  what  I  meant." 

"  Then  I  give  it  up,"  rejoined  John. 

"It  would  injure  my  credit  and  character  for  trustwor- 
thiness and  faithfulness  with  you,"  said  Lawrence.  "  If  you 
found  that  I  would  take  a  thing  from  Flippy  on  certain 
conditions,  understood,  and  then  would  not  observe  the 
conditions  because  he  was  no^ there  to  see,  you  could  never 
have  full  confidence  in  my  faithfully  fulfilling  any  condi- 
tions that  I  should  make  with  yow." 

John  pondered  somewhat  thoughtfully  upon  this  view 
of  the  case,  but  he  did  not  reply.  Indeed,  it  was  pretty 
evident  that  there  was  nothing  that  could  be  very  well 
said  in  reply. 

Lawrence  attached  great  importance  to  the  idea  of  sus- 
taining the  character  of  perfect  trustworthiness  in  the  esti- 
mation of  all  who  knew  him.  He  was  particularly  desirous 
that  John  should  at  all  times  have  entire  confidence  in 
him.  He  knew,  moreover,  that  the  only  sure  way  of  mak- 
ing all  who  know  us  believe  that  we  are  thoroughly  honest 
and  true,  is  to  be  in  reality  thoroughly  honest  and  true, 


QUESTION   ABOUT   A   NAME.  283 


CHAPTER  XXX. 

UP    THE    NORTH    KIVER. 

FOR  twenty  or  thirty  miles  above  New  York,  the  North 
River,  as  it  is  there  called,  is  of  its  ordinary  width,  and 
runs  in  a  pretty  straight  course,  with  a  range  of  lofty  and 
precipitous  cliffs  on  one  side,  and  a  series  of  charming  land- 
scapes, consisting  of  groves,  gardens,  pleasure-grounds,  vil- 
las, public  institutions,  and  pretty  little  landings  leading 
to  them,  on  the  other.  For  an  hour  after  leaving  the  pier 
at  New  York,  Lawrence  and  John  remained  at  their  seats 
upon  the  upper  deck,  in  the  midst  of  many  animated  groups 
formed  of  the  other  passengers — some  talking,  some  read- 
ing, some  sitting  quietly  in  silence,  but  all  enjoying  the 
mild  and  balmy  air  of  the  evening  and  the  beauty  of  the 
scenery. 

"  Why  do  they  call  this  river  the  North  River  in  New 
York,"  asked  John, "  while  every  where  else  it  is  called  the 
Hudson  River  ?" 

"  That  is  certainly  very  singular,"  said  Lawrence. 

"  Even  the  same  people,"  continued  John,  "  call  it  the 
North  River  when  they  are  here,  and  call  it  the  Hudson 
River  when  they  are  in  Boston." 

"  Not  always,"  said  Lawrence. 

"  No,  not  always,"  replied  John  ;  "  but  why  do  they 
ever?  What  is  the  use  of  having  two  names  for  the  same 
river  at  all  ?" 

"It  is  very  common,"  said  Lawrence,  "to  have  two 
names  for  the  same  thing,  to  be  used  indiscriminately; 
but  this  seems  to  be  a  case  where  the  use  of  one  word  or 


284  UP    THE    NORTH    RIVER. 

the  other  depends  in  some  degree  upon  the  place  we  hap- 
pen to  be  in  when  we  use  it.  That's  a  curious  philological 
phenomenon." 

"  Philological  ?"  repeated  John. 

"  Yes,"  replied  Lawrence ;  "  philology  is  the  science  that 
treats  of  the  origin  and  the  meaning  of  words,  and  the 
changes  they  undergo  in  the  spelling  and  the  use  of  them. 
It  is  a  very  curious  subject.  You  will  be  very  much  in- 
terested in  studying  it  one  of  these  days,  when  you  get 
older." 

"  Should  not  I  be  interested  in  it  now  ?"  asked  John. 

"  Perhaps  so,"  replied  Lawrence.  "You  might  try.  You 
might  begin  by  looking  into  the  histories  of  New  York  and 
of  the  early  settlements  of  this  country,  and  see  if  you  can 
find  out  when  and  why  this  river  received  its  two  names, 
and  also  see  if  you  can  think  of  any  other  cases  where  we 
have  two  different  names  for  the  same  thing,  according  to 
the  place  we  happen  to  be  in  when  we  are  speaking  of  it." 

"Do  you  know  of  any  such  cases?"  asked  John. 

"I  know  of  one,"  replied  Lawrence.  "When  we  are  not 
in  the  cars,  we  commonly  call  the  stopping-places  of  the 
trains  depots,  but  when  we  are  in  them  we  call  such  places 
stations.  We  never  ask,  for  example,  when  we  are  travel- 
ing, '  What  depot  is  this  ?'  or  say  that  we  are  going  to  stop 
at  the  next  depot,  but  always  stativn.  And  yet,  when  out 
of  the  cars,  at  a  hotel,  or  in  the  streets  of  a  town,  people 
almost  always  say  depot." 

"  That's  curious,"  said  John ;  "I  wonder  what  the  reason 
is!" 

"I  think  there  must  be  some  reason,  or  at  least  some  ex- 
planation of  such  a  usage,"  replied  Lawrence.  "  It  would 
be  a  good  plan  for  you,  some  time  when  you  have  nothing 
to  do,  to  think  of  it,  and  see  if  you  can  study  it  out." 

Lawrence  did  not  think  it  at  all  necessary  that  he  should 


SELF-CONCEIT  AND   VANITY.  285 

try  to  give  some  kind  of  explanation,  satisfactory  or  other- 
wise, of  every  remarkable  appearance  or  phenomena  which 
they  chanced  to  observe,  especially  when  the  questions 
which  arose  in  connection  with  them  related  to  branches 
of  knowledge  which  John,  in  the  course  of  his  education, 
had  not  yet  reached.  He  was  very  willing  to  open  before 
him,  from  time  to  time,  glimpses  of  fields  of  investigation 
to  the  very  boundaries  of  which  he  had  not  yet  attained. 

There  was  a  double  advantage  in  this.  In  the  first  place, 
the  bringing  to  his  view  in  this  way  curious  and  interest- 
ing questions  connected  with  scenes  which  he  had  not  be- 
gun to  study,  and  of  the  very  nature  of  which  he  had  but 
little  idea,  expanded  his  ideas  in  respect  to  the  vast  extent 
of  the  field  of  knowledge  which  he  had  yet  to  explore,  and 
increased  his  interest  in  going  forward.  Then,  in  the  sec- 
ond place,  showing  him  the  boundlessness  of  the  field  be- 
fore him  tended  to  prevent  his  becoming  vain  and  con- 
ceited in  thinking  of  the  acquisitions  that  he  had  already 
made. 

I  say  only  that  it  tended  to  produce  this  last  good  re- 
sult, for  it  is  almost  impossible  to  accomplish  it  entirely. 
Boys  like  John,  who  take  a  great  interest  in  learning  all 
they  can,  and  who,  of  course,  make  rapid  progress  in  learn- 
ing, almost  always,  for  a  time,  become  more  or  less  conceit- 
ed. It  is  not  at  all  surprising  that  it  should  be  so,  since 
their  appreciation  of  what  is  contained  within  the  little 
field  which  they  have  already  explored  is  necessarily  so 
much  more  vivid  and  distinct  than  any  conceptions  which 
they  can  form  of  what  is  before  them  in  the  boundless  re- 
gions into  which  they  have  not  yet  entered. 

About  twenty  or  thirty  miles  above  New  York  the  river 
expands  into  a  broad  and  spacious  lake,  called  the  Tappan 
Sea. 

"  We  are  coming  to  the  Tappan  Sea,"  said  John.     "  Let 


286  UP   THB    NORTH    EIVER. 

us  go  forward,  so  that  we  can  look  out  ahead  and  see  the 
vessels  on  the  water." 

So  they  rose  from  their  seats  and  walked  through  the 
long  upper  saloon  to  the  forward  part  of  the  steam-boat. 
This  saloon  was  richly  decorated,  carpeted,  and  furnished, 
and  many  groups  of  gentlemen  and  ladies  were  seated 
upon  the  sofas,  and  lounges,  and  comfortable  chairs,  and 
parties  of  children  were  playing  together  here  and  there 
upon  the  floor.  Along  the  sides  of  the  room  were  ranges 
of  doors  opening  into  the  different  staterooms.  Ihe  room 
was  very  long,  and  had  a  very  rich  and  elegant  appear- 
ance, but  the  whole  expression  of  the  interior  was  entirely 
different  from  that  of  the  main  cabins  of  a  sea-going  steam- 
er. There  every  thing  is  solid,  massive,  strong,  and  firmly 
secured ;  here  the  style  was  comparatively  light,  airy,  and 
graceful,  and  to  the  eyes  of  Lawrence  and  John,  accus- 
tomed, as  they  were,  to  the  shocks,  and  concussions,  and 
general  rough  usage  which  the  Scotia  or  the  Cuba  had  had 
to  sustain  from  the  billows  of  the  Atlantic,  seemed  exceed- 
ingly frail. 

From  the  forward  end  of  this  saloon  Lawrence  and  John 
passed  out  through  a  door  to  an  open  part  of  the  deck  over 
the  bows,  where  they  had  a  very  fine  view  of  the  grand  ex- 
panse of  water  before  them. 

"What  a  splendid  lake!"  said  John;  "and  how  many 
steam-boats  and  vessels !" 

"Yes,"  replied  Lawrence;  "isn't  it  a  pity  that  is  all 
going  to  be  filled  up?" 

"Going  to  be  filled  up !"  repeated  John,  much  surprised; 
"  what  are  they  going  to  fill  it  up  for?" 

"They  are  not  going  to  do  it.  It  is  the  river  that 
will  do  it,"  replied  Lawrence.  "  The  river  will  fill  it  all 
up,  except  a  winding  channel  that  it  will  leave  through 
the  land  that  it  makes  for  its  own  flow.  All  the  rest 


FILLING    UP    OF   LAKES.  287 

will  be  filled  up  and  formed  into  a  region  of  level  green 
fields." 

John  was  much  surprised  at  this  statement,  and  asked 
how  it  would  be  done.  Lawrence  explained  to  him  that 
the  lake  was  a  vast  hollow  in  the  land  filled  with  water, 
and  that  the  river  was  all  the  time  bringing  down  sand, 
and  pebbles,  and  sediments  of  various  kinds  from  the  coun- 
try above ;  and  that,  though  some  of  these  materials  were 
carried  through  and  borne  out  through  the  lower  end  of 
the  lake,  and  so  onward  into  the  sea,  some  portion  must 
necessarily  be  left  behind,  and  in  process  of  time  the  whole 
lake  must  be  filled. 

"Nonsense!"  said  John;  "such  a  great  lake  as  this 
could  never  be  filled  in  this  way.  There  would  not  be 
sediment  enough  brought  down  to  fill  it — not  in  a  thou- 
sand years !" 

"  Perhaps  not,"  said  Lawrence ;  "  but  if  the  river  could 
not  fill  it  in  a  thousand  years,  it  might  in  ten  thousand." 

"  No,"  rejoined  John,  "  I  don't  believe  it  would  fill  it 
even  in  ten  thousand." 

"  Then  ten  million,"  replied  Lawrence.  "  You  can  have 
as  many  years  as  you  want.  There  are  plenty  of  them 
coming.  If  there  is  any  deposit  at  all  left  in  the  lake,  and 
nothing  to  take  it  away,  the  lake  must  some  time  or  other 
become  filled  up." 

The  conversation  on  this  subject  was  continued  between 
Lawrence  and  John  for  some  time,  and  in  the  course  of  it 
Lawrence  explained  somewhat  at  length  the  manner  in 
which  natural  depressions  in  the  surface  of  the  land  which 
occur  in  the  course  of  the  current  of  a  river,  or  wridenings 
of  the  valley  through  which  it  flows,  and  which  at  first  be- 
come, of  course,  so  many  reservoirs  of  water  supplied  by 
the  river,  thus  forming  lakes,  are  gradually  filled  up  by  de- 
posits of  sand  and  soil,  so  as  to  form  in  the  end  broad  plains 


288  UP    THE    NORTH    KIVEK. 

bordering  the  river,  covei'ed  with  verdure  and  trees,  and 
with  a  tortuous  channel  through  the  centre  of  them  kept 
open  for  the  passage  of  the  water. 

The  process  is  a  very  curious  one,  and  has  been  observed, 
and  the  different  steps  of  the  progress  of  it  in  particular 
instances  have  been  carefully  noted  and  recorded  by  men 
of  science. 

The  philosophy  of  the  operation  is  this :  All  rivers  in 
their  flow  bring  down  with  them  a  great  deal  of  sediment- 
ary matter,  which  results  in  part  from  the  disintegration 
of  the  rocks  and  mountains  among  which  their  several 
branches  take  their  rise,  and  also  from  dust  blown  into 
them  by  the  wind,  and  from  decayed  animal  and  vegetable 
substances  brought  into  them  by  the  rains. 

The  heavier  portions  of  these  substances  sink  rapidly, 
and  are  rolled  along  the  bottoms  of  the  rivers  in  the  form 
of  pebbles  and  sand.  Those  that  are  not  so  heavy  sink 
more  slowly,  and  where  the  flow  of  the  stream  is  rapid  and 
turbulent,  their  complete  subsidence  is  entirely  prevented 
by  the  surging  and  whirl  of  the  water;  and  in  general, 
the  tendency  to  subsidence  on  the  part  of  the  solid  matter 
held  in  suspension  is  determined  in  a  great  measure  by  the 
slowness  or  swiftness  of  the  current. 

Now,  in  all  those  places  where  the  river  is  very  broad 
and  deep,  the  motion  of  the  water  is  very  slow,  on  account 
of  the  space  through  which  it  moves  being  so  vast,  and  the 
quantity  moving  being  so  great,  that  the  whole  amount 
that  has  to  pass  through  during  a  given  time  can  be  trans- 
mitted by  a  very  slow  motion. 

Of  course,  in  all  those  places  where  the  space  is  so  wide 
and  deep  as  to  form  a  lake,  the  deposition  takes  place  much 
more  rapidly  than  in  other  places ;  and,  unless  something 
interferes  with  the  process,  the  lake,  after  a  certain  time, 
becomes  entirely  filled  up. 


CHANNEL   KEPT   OPEN.  289 

"  Then  I  don't  see,"  said  John,  when  Lawrence  had  ar- 
rived at  this  point  in  his  explanation,  "how  any  channel  is 
left  for  the  passage  of  the  water." 

"  There  is  something  very  curious  and  remarkable  about 
that,"  replied  Lawrence.  "  You  see  that  the  tendency  to 
deposit  is  greatest  where  the  water  is  most  nearly  in  a 
state  of  repose,  and  least  along  the  line  of  swiftest  motion. 
Where  this  line  of  swiftest  motion  would  be  would  depend 
much  upon  the  conformation  of  the  shores,  but  it  would  in 
general  tend  to  pass  somewhere  through  the  middle  of  the 
lake.  Of  course,  as  the  progress  of  the  deposition  goes  on 
nearer  the  shores  and  in  all  the  stiller  portions  of  the 
water,  the  space  which  the  whole  volume  of  the  water  will 
have  for  its  flow  will  be  more  and  more  contracted,  and 
the  current  along  it  will  become  swifter  and  swifter,  and 
thus,  as  the  channel  becomes  contracted  and  defined,  there 
will  be  an  increasing  force  in  the  flow  of  the  water  to  keep 
it  from  being  closed  entirely. 

"  At  last,"  continued  Lawrence, "  things  would  come  in 
such  a  case  into  a  state  of  equilibrium — that  is,  the  tenden- 
cy of  the  sediment  to  subside  through  the  water  by  its 
weight,  and  to  be  borne  onward  by  the  swiftness  of  the 
current,  would  balance  each  other,  and  the  channel  of  the 
river  would  then  become  in  some  measure  permanent  as  to 
its  size — that  is,  as  to  what  is  called  the  area  of  its  section, 
only  now,  instead  of  forming  a  lake,  it  would  flow  mean- 
deringly through  a  level  plain,  over  which  every  freshet 
.vould  deposit  a  fresh  layer  of  fertilizing  soil,  until  it  was 
raised  far  above  the  level  of  the  ordinary  flow  of  the 
river." 

Lawrence  went  on  farther  to  explain  that  this  process  of 

filling  up  all  the  natural  depressions  in  the  land  through 

which  rivers  flow,  and  which  originally  formed  the  beds 

of  lakes,  had  been  going  on  for  thousands  of  years,  and 

N 


290  UP   THE    NORTH    EIVER. 

that  there  were  now  found  along  the  courses  of  all  rivers  a 
great  many  places  where,  according  to  every  appearance, 
there  had  formerly  been  depressions  which  the  river  orig- 
inally filled  with  water,  so  as  to  form  lakes  and  ponds,  but 
which  are  now  filled  up  nearly  to  the  height  of  the  highest 
freshets,  and  have  become  smooth  and  level  plains,  covered 
with  grass  and  trees.  Such  grounds  as  these  are  called 
meadows  and  intervals,  and  sometimes  river  bottoms.  The 
river  flows  through  these  fluviatile  lands — that  is,  river- 
made  lands,  by  a  very  devious  and  winding  channel,  which 
is  continually  changing. 

"  Why  does  not  it  flow  straight,  and  keep  always  to  the 
same  channel  ?"  asked  John. 

"  Ah  !  that  is  a  very  important  question,"  replied  Law- 
rence, "  though  I  have  not  time  to  explain  the  case  to  you 
now,  for  it  is  about  time  for  the  gong  to  sound  for  tea. 
We  shall  have  an  excellent  opportunity  to  study  the  op- 
eration of  the  water  in  a  river  channel  at  Carlton,  when  we 
get  there,  for  you  remember  the  river  twists  and  winds 
about  there  through  the  meadows  in  front  of  our  house, 
and  wears  away  the  banks  on  one  side  or  the  other  inces- 
santly." 

"Yes,"  replied  John,  "it  twines  about  in  great  sweeps, 
and  the  banks  in  the  hollow  of  the  sweeps  are  caving  in." 

"  It  is  almost  always  so,"  rejoined  Lawrence, "  with  the 
course  of  a  river  through  the  lands  which  it  has  made 
itself.  There  is  a  splendid  opportunity  to  see  this  from 
the  top  of  Mount  Holyoke,  where  we  look  down  upon  a  re- 
gion which  seems  once  to  have  been  a  great  lake,  but 
which  now  consists  of  a  plain  formed  of  the  most  fertile 
and  beautiful  meadows  in  the  world,  the  river  flowing 
through  them  with  the  most  extraordinary  windings." 

The  engraving  gives  us  a  glimpse  of  these  lands,  and  of 
the  windings  of  the  river  through  them,  as  seen  from  near 


MOUNT   HOLYOKE.  293 

the  summit  of  Mount  Holyoke.  It  presents  to  our  view  a 
very  perfect  example  of  an  ancient  lake  filled  up,  and  the 
river  flowing  through  the  new  ground  in  a  tortuous  chan- 
nel. 

"  Are  we  going  by  Mount  Holyoke  on  our  way  home  ?" 
asked  John. 

"  We  are  going  pretty  near  to  it,"  said  Lawrence. 

"  Then  let  us  stop  and  go  up,"  said  John ;  "  I  like  to 
climb  mountains  and  see  the  views." 

"  Very  well,"  replied  Lawrence.  "  The  view  from  Mount 
Holyoke  is  very  beautiful,  and  it  is  very  instructive,  too, 
for  one  who  is  studying  these  subjects.  But  we  can  see 
the  operation  of  the  process  to  better  advantage  at  Carl- 
ton,  for  there  every  thing  is  on  a  smaller  scale,  and  the 
changes  are  more  perceptible.  The  great  principles  are 
the  same  in  all  cases,  from  the  smallest  brooks  to  the 
mightiest  rivers.  But  why  does  not  the  gong  sound  ?" 

"  I  wish  it  would  sound,"  replied  John, "  for  I'm  hungry 
for  supper." 

"  The  general  principle  is  this,"  resumed  Lawrence,  re- 
verting to  the  subject  of  the  flow  of  rivers :  "  The  true  and 
ultimate  function  of  brooks  and  rivers  is  to  remove  the 
mountains  to  the  sea  !  Of  course  they  can  not  carry  them 
down  whole,  but  the  frost,  and  the  ice,  and  the  rain  disin- 
tegrate and  wear  away  the  rocks,  and  deliver  the  materials 
into  the  streams  in  such  a  form  that  the  water  can  carry 
them  on.  The  river  first  employs  these  materials  in  filling 
up  all  the  hollows  and  depressions  in  the  ground  that  it 
meets  with  on  its  way.  But  it  does  not  leave  any  single 
portion  of  them  long  there,  for,  by  twisting  and  winding  in 
its  course,  it  continually  washes  away  and  carries  down 
the  stream  successive  portions  of  the  land  it  formed  years 
before,  and  replaces  what  is  thus  removed  from  one  side  of 
the  river  by  new  formations,  which  it  gradually  builds  up 


294  UP   THE    NORTH    KIVER. 

on  the  other  side  from  fresh  materials.  We  shall  be  able 
to  see  all  this  work  going  on,  upon  a  comparatively  small 
scale,  when  we  get  to  Carlton." 

Carlton  was  the  name  which  I  give  to  the  town  where 
Lawrence  and  John  lived.  It  was  situated  among  the 
mountains  in  the  interior  of  New  England. 

"  I  mean  to  watch  the  river  when  I  get  home,"  said  John, 
"  and  see  how  it  works." 

"You  can  even  do  more  than  that,"  rejoined  Lawrence ; 
"  you  can  actually  experiment  with  a  stream  yourself,  if 
you  take  one  small  enough ;  for  the  laws  which  govern  the 
flow  of  water,  and  the  transportation  of  solid  matter  sus- 
pended in  it,  or  borne  along  by  it,  are  the  same,  and  the 
effects  that  result  are  analogous,  whatever  is  the  size  of 
the  stream;  only  in  the  smaller  streams  the  changes  are 
more  rapid,  and  being,  moreover,  comprised  within  a  nar- 
rower area,  are  more  easy  of  observation." 

"  Yes,"  said  John, "  there's  the  Beaver  Brook,  where  I 
used  to  have  my  dam.  I  mean  to  go  and  see  how  it  is  on 
the  Beaver  Brook  as  soon  as  I  get  home." 

The  conversation  on  this  subject  was  here  suddenly  in- 
terrupted by  the  sound  of  the  gong,  on  hearing  which  John 
rose  at  once  with  great  alacrity,  and,  followed  by  Lawrence, 
went  down  to  supper.  He,  however,  did  not  forget  what 
Lawrence  had  explained  to  him  about  the  action  of  rivers 
in  filling  up  such  natural  depressions  in  the  land  as  came 
in  their  course,  and  forming  green  and  fertile  meadows  in 
the  places  they  had  occupied,  nor  the  resolution  which  he 
had  made  to  investigate  the  subject  by  observations  and 
experiments  upon  the  streams  in  the  neighborhood  when 
he  should  reach  home.  An  account  of  the  results  of  these 
observations  and  experiments  will  be  contained  in  the  next 
volume  of  this  series,  which  is  to  be  entitled  WATER  AND 
LAND. 


AT  THE   SUPPEK-TABLE.  295 


CHAPTER  XXXI. 

LIGHTING   BY   GAS. 

THE  sun  had  gone  down  and  the  twilight  was  far  ad- 
vanced before  the  gong  was  sounded  which  summoned  the 
passengers  on  board  the  steamer  to  supper,  and  when  Law- 
rence and  John  went  below  they  found  the  supper-tables 
lighted  by  a  long  row  of  candles. 

"Why  don't  they  light  the  cabin  with  gas?"  asked  John, 
as  soon  as  they  were  seated  at  the  table.  "  Oh  !  I  might 
have  known  myself,"  he  added,  after  a  moment's  reflec- 
tion ;  "  they  could  not  bring  the  pipes  on  board." 

"  True,"  replied  Lawrence,  "  they  could  not  bring  the 
gas  in  by  pipes  from  the  mains  in  the  city,  but  there  are 
other  ways  in  which  we  can  conceive  of  gas  being  brought 
on  board  a  steamer  besides  drawing  it  from  the  great  city 
gasometers.  In  the  form  in  which  it  exists  in  these  gasom- 
eters, it  is  altogether  too  much  expanded  and  too  bulky  to 
be  conveniently  transported  or  stored,  but  there  are  two 
modes  of  bringing  it  in  a  more  compact  form :  first,  by  in- 
troducing it  in  what  may  be  called  the  original  packages, 
and,  secondly,  by  packing  it  anew  expressly  for  the  pur- 
pose." 

John  did  not  know  at  all  what  Lawrence  meant  by  this 
language.  He  did  not  understand,  he  said,  how  such  a  sub- 
stance as  gas  could  be  packed  at  all.  So  Lawrence  ex- 
plained to  him  what  he  meant.  He  did  this  in  conversa- 
tion which  was  partly  held  at  the  supper-table,  and  partly 
afterward  in  the  saloon  above,  when  they  went  up  after 
the  supper  was  concluded.  The  substance  of  the  conver- 
sation was  this : 


296  LIGHTING   BY   GAS. 

One  would  not  suppose  that  such  a  substance  as  gas 
could  be  packed  very  easily  in  any  way,  and  yet  Nature 
has  the  art  of  stowing  it  in  a  very  compact  form  in  all  that 
class  of  substances  which  have  already  been  described  as 
hydrocarbons — that  is  to  say,  in  almost  all  natural  sub- 
stances that  are  inflammable.  It  is  packed  very  closely 
in  wood,  in  all  bituminous  coal,  and  in  all  such  substances 
as  resin,  pitch,  wax,  and  tallow. 

"Indeed,"  said  Lawrence,  pointing  to  one  of  the  tall  can- 
dles which  stood  upon  the  table  before  them  while  they 
were  at  supper,  "  providing  these  candles  is  only  a  mode 
of  bringing  gas  on  board  in  a  compact  and  manageable 
form.  The  paraffine  of  which  these  candles  are  made  is  a 
hydrocarbon — that  is,  it  is  composed  chiefly  of  hydrogen 
and  carbon  combined  with  each  other  and  packed  very 
closely  together.  The  heat  of  the  burning  wick  liberates 
them  and  restores  them  to  their  gaseous  form,  and  they 
then  burn,  just  as  the  gas  in  the  cities  does  from  a  jet; 
only,  in  the  case  of  the  candle,  the  gas  is  burned  directly 
as  fast  as  it  is  set  free,  and  in  the  place  where  it  is  set  free, 
instead"  of  being  saved  and  stored  in  a  great  reservoir,  and 
then  conveyed  in  pipes  to  be  burned  in  different  places  at 
a  distance  from  where  it  is  produced.  In  a  philosophical 
point  of  view,  and  in  all  essential  respects,  the  burning  of 
a  candle  is  the  same  as  burning  gas  from  a  jet." 

"  That's  curious,"  said  John ;  "  and  is  it  the  same  with 
a  lamp  ?" 

"  Precisely  the  same,"  replied  Lawrence ;  "  only,  in  the 
case  of  the  lamp,  the  material  from  which  the  gas  is  dis- 
tilled is  a  liquid,  instead  of  being  a  solid,  as  it  is  in  the 
case  of  the  candle. 

"  Thus,  in  point  of  fact,"  continued  Lawrence,  "  they  do 
burn  gas  in  this  steamer.  They  bring  it  on  board  packed 
very  snugly  in  the  paraffine  of  the  candles.  They  might, 


NATURAL  CONDENSATION.  297 

even,  in  fact,  bring  it  packed  in  coal,  were  it  not  for  the  in- 
convenience they  would  incur  in  that  case  in  the  work  of 
unpacking  it." 

In  speaking  thus  of  hydrogen  and  carbon,  which  are  the 
constituents  of  illuminating  gas,  as  packed  in  paraffine  and 
in  coal,  Lawrence  used  language,  it  must  be  confessed,  in  a 
somewhat  figurative  sense ;  but  these  materials  do  certain- 
ly exist  in  these  substances  in  a  very  highly  condensed  and 
concentrated  condition.  Indeed,  Nature  seems  to  have  the 
power  of  carrying  into  eifect  this  kind  of  packing  in  a  most 
extraordinary  degree. 

Water,  for  example,  is  composed  of  the  two  substances 
oxygen  Snd  hydrogen,  both  of  which  in  their  ordinary  con- 
dition, as  known  to  us,  appear  in  the  form  of  a  gas.  Na- 
ture, in  combining  these  substances  in  the  form  of  water, 
brings  enormous  volumes  of  them  into  very  small  compass, 
and  retains  them  in  that  condition  without  any  external 
force  of  compression  or  any  means  of  confinement  what- 
ever. Man  can  not  produce  this  condensation  by  a  press- 
ure of  a  hundred  and  fifty  atmospheres. 

I  shall  presently  explain  what  is  meant  by  an  atmosphere 
as  a  measure  of  pressure,  though  the  explanation  will  not 
help  the  reader  to  form  any  distinct  conception  of  what  a 
pressure  of  a  hundred  and  fifty  atmospheres  is,  as  no  one 
can  form  any  adequate  idea  of  such  enormous  forces  ex- 
cept those  who  have  witnessed  the  production  of  them 
and  observed  practically  some  of  their  effects. 

Somewhat  in  the  same  way  by  which  the  powers  of  na- 
ture hold  the  naturally  gaseous  substances  of  oxygen  and 
hydrogen  in  so  very  compact  and  concentrated  a  condition 
in  water,  do  they  also  hold  the  carbureted  hydrogen  in  the 
paraffine  of  the -candle  and  in  coal.  In  the  case  of  coal,  the 
quantity  held  within  a  given  space  varies  much,  according 
to  the  different  qualities  of  the  coal,  and  to  other  circunv 
N2 


298  LIGHTING    BY    GAS. 

stances ;  but  it  is  not  uncommon  to  find  a  quantity  of  il- 
luminating gas  sufficient  to  fill  a  room  thirty  feet  square 
and  ten  feet  high  so  closely  compressed  in  the  coal  con- 
taining it,  that  if,  while  it  was  in  that  state,  it  could  be 
separated  from  the  other  constituents  of  the  coal,  it  would 
form  a  solid  block  which  a  man  could  easily  lift. 

Thus,  as  Lawrence  said,  bringing  the  gas  on  board  the 
vessel  packed  in  paraffine  or  in  coal  is  altogether  a  more 
convenient  mode  than  to  attempt  to  bring  it  in  pure,  in  its 
natural  form  and  of  its  natural  bulk,  as  gas.  In  the  form 
of  paraffine  it  is  much  more  expensive,  in  the  first  instance, 
than  as  one  of  the  constituents  of  coal,  but  then  it  is  much 
more  easily  extracted,  or,  perhaps,  it  would  be  Better  to 
say,  developed,  from  that  substance  than  from  coal ;  for, 
in  the  case  of  paraffine,  or  wax,  or  tallow,  or  any  other  such 
substance,  all  that  is  necessary  is  to  have  a  wick  passing 
up  through  it  and  set  on  fire,  and  the  process  of  melting 
successive  portions  of  the  substance,  and  converting  them 
into  an  illuminating  gas,  goes  on  of  itself,  without  any  ap- 
paratus or  machinery  whatever. 

Whereas,  on  the  other  hand,  although  people  might  ob- 
tain the  necessary  supply  of  gas  in  coal  cheaper  than  in 
any  of  those  other  forms,  there  would  be  required  a  com- 
plicated, and  expensive,  and  bulky,  and  even  somewhat 
dangerous  apparatus  to  distill  it.  There  would  have  to  be 
a  furnace  to  heat  the  coal,  and  tight  iron 'retorts  to  contain 
it  so  as  to  prevent  the  gas  from  being  burned  in  the  fur- 
nace as  fast  as  it  was  produced,  and  a  reservoir  to  store  it, 
and  pipes  to  convey  it  to  the  different  parts  of  the  vessel 
ivhere  it  might  be  required,  all  of  which  would  involve 
much  trouble  and  expense. 

"  That  would  not  do  at  all,"  said  John,  when  Lawrence 
explained  these  things  to  him. 

"Especially,"  he  added,  after  thinking  a  moment,  "in 


THE    LIGHT    OF   LAMPS    AND   CANDLES.  299 

the  case  of  a  steamer  at  sea,  tossing  and  pitching  about  in 
a  storm." 

Besides  these  objections  which  Lawrence  pointed  out, 
we  may  add  that  the  process  of  preparing  gas  from  coal, 
or,  as  Lawrence  called  it,  the  work  of  "  unpacking  it,"  not 
only  involves  the  use  of  complicated  machinery,  but  re- 
quires skilled  workmen  to  manage  the  machinery  and  to 
conduct  the  process.  And  these  men  must  devote,  too,  all 
their  time  to  the  work,  and  must  be  well  paid,  so  that  it 
is,  on  every  account,  much  better  to  produce  the  gas  for 
illumination  from  some  of  the  substances  that  can  be  used 
in  the  form  of  candles  or  in  lamps,  though  they  cost  more 
at  the  outset.  It  is  only  when  very  large  quantities  of  gas 
are  required,  and  in  places,  too,  where  there  is  ample  room 
for  all  the  machinery  and  appointments,  that  it  can  be 
profitably  obtained  from  coal. 

Thus  it  can  be  manufactured  advantageously  on  a  great 
scale  for  lighting  cities  and  towns,  and  even  for  extensive 
private  establishments  where  there  is  plenty  of  space  at 
command  for  the  necessary  works;  but  for  single  dwell- 
ings, or  small  establishments  of  every  kind,  if  they  are  to 
be  lighted  artificially  at  all,  the  gas  must  be  brought  in 
packed,  as  Lawrence  called  it,  in  paraffine,  or  wax,  or  sper- 
maceti, or  tallow,  or  oil,  or  kerosene,  or  some  other  similar 
hydrocarbon. 

"I  never  thought  before,"  remarked  John,  when  Law- 
rence had  made  these  explanations  to  him,  "  that,  when 
we  were  burning  lamps  or  candles,  we  were  really  burning 
gas." 

"Yes,"  replied  Lawrence;  "what  is  actually  burnt  in 
both  cases  is  essentially  the  same,  only,  in  the  case  of  a 
candle  or  lamp,  the  gas  is  burned  as  fast  as  it  is  set  free, 
while  in  the  case  of  regular  gas-works  it  is  kept  from  be- 
ing burned  for  a  time  after  it  is  set  free,  and  is  conveyed 


300  LIGHTING    BY   GAS. 

in  pipes  wherever  the  light  from  it  is  wanted.  Even  the 
flame  of  burning  wood  from  a  fire  is  the  flame  of  gas,  you 
recollect." 

"  I  remember  you  told  me  once,"  replied  John,  "  how  I 
might  draw  it  off  from  the  fire  through  a  pipe-stem,  and 
burn  it  at  the  end  of  the  stem." 

"Yes,"  rejoined  Lawrence;  "and  we  might  easily  draw 
it  off  farther  than  that,  if  we  chose,  by  means  of  an  India- 
rubber  tube. 

"  Only,  in  that  case,"  added  Lawrence,  "  it  would  be  bet- 
ter to  take  some  other  larger  and  stronger  receptacle  than 
the  bowl  of  a  pipe  for  a  retort — a  gun-barrel,  for  instance. 
Chemists  employ  gun-barrels  very  often  for  such  experi- 
ments. An  old  gun-barrel  which  is  past  service  for  shoot- 
ing, such  as  can  generally  be  obtained  at  a  gunsmith's,  will 
make  a  very  good  retort  for  such  purposes." 

Lawrence  went  on  to  explain  that,  by  taking  such  a  gun- 
barrel,  and,  after  plugging  up  the  touch-hole,  filling  it  half 
full  of  some  hydrocarbon  and  connecting  a  long  India-rub- 
ber tube  with  the  outer  end  of  it,  the  gas  could  be  con- 
veyed away  to  any  distance — to  a  stand  of  some  kind,  for 
example,  upon  a  table  in  the  middle  of  a  room — and  there 
burned  just  like  gas  from  a  pipe  laid  in  the  street. 

John  said  that  he  should  like  very  much  to  see  that 
done. 

"Very  well,"  replied  Lawrence ;  "  we  can  do  it,  or,  rath- 
er, you  can  do  it  yourself  under  my  direction,  when  we  get 
home.  I  mean  to  fit  up  a  little  laboratory  and  workshop 
in  Carlton,  and  you  can  then  perform  as  many  such  ex- 
periments as  you  like." 

"  I  mean  to  make  some  gas,  at  any  rate,  for  one  thing," 
replied  John.  "  Only,"  he  added,  after  reflecting  a  mo- 
ment, "  I  should  think  that  the  end  of  the  India-rubber 
tube,  where  it  is  slipped  over  the  end  of  the  gun-barrel, 


KEEPING  THE   IKON  COOL.  301 

would  begin  to  melt  or  burn  pretty  soon.  You  see,  if  the 
butt  end  of  the  barrel  was  in  the  burning  coals,  the  muzzle 
end  would  get  quite  hot  in  a  very  short  time." 

"  Certainly,"  replied  Lawrence, "  unless  we  devised  some 
way  to  keep  it  cool.  There  are  a  great  many  practical  dif- 
ficulties of  this  kind  to  be  encountered  in  making  chemical 
experiments,  and  it  requires  sometimes  a  good  deal  of  in- 
genuity to  contrive  means  to  surmount  them.  That  is  one 
reason  why  making  chemical  experiments  is  so  useful  to  a 
boy  so  soon  as  he-  is  old  enough  for  such  work.  It  sharp- 
ens his  wits. 

"  As  to  keeping  the  end  of  the  gun-barrel  cool,"  contin- 
ued Lawrence, "  there  is  a  simple  mode  of  doing  that.  We 
have  only  to  wrap  the  outer  end  of  the  barrel,  where  the 
India-rubber  tube  joins  it, 'with  a  strip  of  cotton  cloth, 
winding  it  round  and  round  in  the  form  of  a  bandage,  and 
then  keeping  the  cloth  wet  by  pouring  on  a  little  water 
from  time  to  time  out  of  a  pitcher." 

"  Yes,"  replied  John, "  that  would  keep  it  cool." 

"  Water  has  a  wonderful  power  to  keep  any  thing  cool," 
said  Lawrence, "  even  though  it  is  hot  water." 

"  That  is  very  strange,"  said  John. 

"  I  mean,"  said  Lawrence, "  to  keep  any  thing  from  get- 
ting very  hot — red  hot,  or  hot  enough  to  melt  or  bum 
India-rubber,  for  example;  for,  before  the  iron  around 
which  the  wet  cloth  is  bound  becomes  hot  enough  for  that, 
it  will  be  hot  enough  to  boil  the  water,  and  water  absorbs 
such  an  enormous  quantity  of  heat  in  boiling  as  to  keep 
the  temperature  of  the  iron  down  to  a  comparative  low 
point.  Of  course,  as  fast  as  the  water  in  the  cloth  is  boiled 
and  converted  into  steam,  you  must  pour  on  more,  so  as  to 
keep  the  cloth  all  the  time  wet." 

u  That  would  be  a  great  deal  of  trouble,"  said  John. 

"  Yes,"  replied  Lawrence, "  and  there  would  be  a  great 


302  .LIGHTING    BY   GAS. 

many  other  troubles  and  inconveniences  in  the  attempt  to 
produce  gas  on  a  small  scale  for  any  practical  purposes, 
but  we  might  be  willing  to  take  the  trouble  once  for  the 
sake  of  performing  the  experiment." 

"  Oh  yes,"  replied  John ;  "  and  I  mean  to  try  it  if  you  will 
help  me.  I  mean  to  have  a  small  pitcher  and  pour  a  little 
water  on  every  few  minutes." 

Before  leaving  this  subject  of  the  management  of  gas,  I 
will  add  that  there  is  an  artificial  mode  of  packing  this 
bulky  commodity  after  it  is  evolved,  by  compressing  it, 
with  great  force,  in  metallic  reservoirs  made  prodigiously 
strong  to  resist  the  pressure.  The  gas  is  driven  into  these 
reservoirs  by  means  of  forcing-pumps  working  with  great 
power.  The  French  have  adopted  this  system  in  Paris  to 
a  considerable  extent.  The  engraving  represents  a  wagon 
loaded  with  gas  thus  compressed. 

The  interior  of  the  wagon  is  occupied  by  nine  cylinders, 
which  are  made  of  copper,  and  are  of  enormous  strength. 
There  is  forced  into  each  cylinder  ten  or  twelve  times  as 
much  gas  as  it  would  naturally  contain  if  the  gas  were  of 
its  ordinary  density;  and  as  the  expansive  pressure  of  the 
gas  is  in  proportion  to  the  quantity  of  it  that  is  forced  into 
a  given  space,  the  whole  interior  surface  of  each  cylinder 
has  a  force  pressing  upon  it  from  within  outward,  and  so 
tending  to  burst  it,  often  or  twelve  atmospheres! 

For  you  must  understand  that  pressure  in  mechanics  is 
measured  by  atmospheres.  The  pressure  of  the  atmosphere 
is  reckoned  at  fifteen  pounds  to  the  square  inch.  The 
actual  pressure  of  the  atmosphere  varies  from  day  to  day 
in  the  same  place  according  to  the  quantity  of  air  that 
there  may  happen  to  be  for  the  time  being  over  the  place, 
and  in  different  places  according  to  their  elevation  above 
the  level  of  the  sea ;  but  fifteen  pounds  to  the  square  inch 
is  taken  as  the  standard  of  measurement,  or,  in  other  words, 


MEASUREMENT   OF   PRESSURE.  305 

fifteen  pounds  to  the  square  inch  is  an  atmosphere  of  press- 
ure. This  is  a  fundamental  principle  or  fact  which  it  is 
very  important  to  remember.  It  comes  continually  into 
philosophical  and  mechanical  calculations. 

The  meaning  of  the  principle  thus  stated  is,  that,  light 
and  rare  as  the  atmosphere  seems  to  us,  in  moving  through 
it,  it  extends  to  so  great  a  height  above  the  surface  of  the 
earth,  and  the  quantity  is  in  the  whole  so  great,  that  the 
weight  of  it  is  equal  to  fifteen  pounds  upon  every  square 
inch  that  it  presses  upon.  That  is  to  say,  if  you  place  a 
small  block  of  wood  an  inch  square  upon  a  table,  and  a  fif- 
teen pound  weight  upon  the  block,  the  additional  pressure 
would  be  that  of  one  atmosphere;  and  this  additional  press- 
ure would  be  just  equal  to  the  original  pressure  of  the  at- 
mosphere itself,  so  that,  with  the  pressure  of  the  weight,  the 
whole  pressure  would  be  exactly  doubled. 

Now  this  original  pressure,  great  as  it  is,  is  not  felt  by 
us,  because  it  acts  in  every  direction ;  just  as  a  fish  swim- 
ming in  the  water  does  not  feel  the  weight  of  the  water 
over  him,  because,  the  water  being  so  perfect  a  fluid,  the 
pressure  resulting  from  the  weight  diffuses  itself  and  bal- 
ances itself  in  every  direction,  so  that  the  fish  floats  in  it, 
as  it  were — in  the  pressure,  I  mean,  not  the  water — and  is 
not  sensible  of  it  at  all ;  so  wre  ourselves  float,  as  it  were, 
in  the  pressure  of  the  air,  which  acts  from  above  and  be- 
low, upon  every  substance  and  upon  every  side  of  it,  equal- 
ly, and  even  from  the  pores  and  interstices  within  it  out- 
wardly, so  that  it  produces,  in  ordinary  cases,  no  percepti- 
ble effect.  The  amount  of  it,  however,  in  every  direction 
and  from  every  side,  is  fifteen  pounds  to  the  square  inch. 

Now  this  pressure  is  really  much  greater  than  one  would 
at  first  imagine.  The  surface  of  one  side  of  a  man's  hand, 
for  example,  contains  not  less,  including  the  fingers,  than 
twenty  square  inches;  consequently,  the  weight  of  the  air 


306  LIGHTING    BY    GAS. 

pressing  upon  the  hand  when  the  man  holds  it  out  hori- 
zontally before  him  is  not  less  than  twenty  times  fifteen 
pounds — that  is,  three  hundred  pounds !  Now,  if  this  down- 
ward pressure  upon  the  upper  surface  were  not  balanced 
and  counteracted  by  an  equal  upward  pressure  upon  the 
under  surface,  and  also  from  certain  resisting  pressures  ex- 
erted by  the  fluids  within  the  hand,  no  man  could  hold  his 
hand  out  horizontally  in  that  manner  for  a  moment. 

We  see  what  the  prodigious  force  of  this  pressure  is, 
when  not  counterbalanced,  by  the  action  of  certain  steam- 
engines  which  are  worked  on  the  principle  of  producing  a 
vacuum  upon  each  side  of  the  piston  in  the  cylinder  alter- 
nately, by  which  means  the  counterbalancing  pressure  is 
taken  off,  and  the  pressure  of  the  air  on  the  other  side  is  al- 
lowed to  act  without  any  thing  to  oppose  it.  By  this  means 
the  piston  is  driven  to  and  fro  with  prodigious  force,  de- 
veloping a  power  that  is  sufficient  to  work  the  heaviest  ma- 
chinery, and  all  by  the  simple  pressure  of  the  atmosphere 
upon  one  side  of  the  piston  when  the  balancing  resistance 
on  the  other  side  is  taken  away. 

And  yet  this  is  only  the  pressure  of  one  atmosphere.  It 
is  precisely  this  amount  of  pressure  which  is  exerted  both 
on  the  inside  and  on  the  outside  of  a  glass  bottle,  or  any 
other  receptable  the  interior  of  which  has  an  open  and  free 
communication  with  the  outside  air. 

If,  however,  this  free  communication  is  closed,  and  a 
double  quantity  of  air  is  forced  into  the  receptacle 
through  a  pipe  fitted  to  it  for  the  purpose,  then  we  should 
have  the  pressure  of  two  atmospheres  on  the  inside  and 
only  one  on  the  outside,  and  there  would  be  a  surplus 
force  of  fifteen  pounds  upon  every  square  inch  of  the  in- 
ternal surface,  tending  to  burst  the  vessel.  If  treble  the 
quantity  were  introduced,  then  there  would  be  a  prepon- 
derance of  two  atmospheres — that  is,  of  thirty  pounds  to 


GAS   COMPRESSED.  307 

the  square  inch.  This  amount  of  pressure  on  every  square 
inch  of  a  vessel  of  the  size  of  a  barrel,  for  example,  would 
constitute  an  enormous  bursting  force— af  force  of  forty  or 
fifty  thousand  pounds ! 

It  is  on  this  principle,  however,  that  the  copper  cylinders 
in  the  gas  wagon  shown  in  the  engraving  were  filled,  and 
yet  so  prodigiously  heavy  and  strong  were  they  made,  that 
sometimes,  as  has  already  been  said,  ten  or  twelve  volumes 
of  gas  were  forced  into  them.  If  the  number  is  taken  as 
eleven,  then,  allowing  one  to  balance  the  ordinary  atmos- 
pheric pressure  on  the  outside,  we  should  have  an  expan- 
sive force  of  a  hundred  and  fifty  pounds  to  the  square  inch 
acting  all  the  time  upon  the  interior  surfaces  of  all  the  cyl- 
inders. 

The  gas,  in  this  compact  form,  was  conveyed  about  the 
city  and  delivered  to  the  consumers.  Those  who  chose  to 
take  their  gas  of  this  company,  of  course,  were  obliged  to 
provide  the  means  of  receiving  it  in  the  form  of  a  gasome- 
ter, or  of  some  very  strong  and  well-secured  receptacle,  for 
the  cylinders  in  the  wagon  were  altogether  too  massive, 
solid,  and  heavy  to  be  removed.  When  the  wagon  arrived 
at  the  door  of  one  of  the  customers,  a  pipe  from  one  of  the 
cylinders  in  the  wagon  was  connected  with  one  communi- 
cating with  the  reservoir  within,  and  then,  when  the  stop- 
cock was  open,  the  gas  from  the  cylinder  would  rush  in  by 
its  own  expansive  force  until  the  quantity  in  the  two  re- 
ceptacles was  equal — that  is,  in  case  the  receptacles  them- 
selves were  equal — and  the  pressure  would  be  that  of  five 
atmospheres  in  each. 

Then,  of  course,  no  more  would  flow  from  that  cylinder, 
but  an  additional  quantity  could  be  thrown  in  from  a  fresh 
cylinder  where  the  pressure  of  the  whole  ten  atmospheres 
was  still  entire.  The  first  cylinder,  moreover,  which  had 
delivered  half  its  gas,  could  be  made  to  deliver  more  at 


308  LIGHTING    BY   GAS. 

the  establishment  of  the  next  customer,  whose  receptacle, 
being  empty,  would  be  ready  to  take  half  of  that  which 
still  remained.  Thus,  while  the  cylinder  in  the  wagon 
would  deliver  five  atmospheres  at  the  first  customer's,  it 
would  deliver  two  and  a  half,  which  would  be  half  of  the 
remainder,  at  the  second,  and  so  on.  In  this  way,  with 
proper  management,  a  large  portion  of  the  load  could  be 
delivered,  and  the  residue,  which  was  not  delivered,  would 
not  be  lost,  but  would  remain  in  the  cylinders  as  so  much 
toward  the  next  filling. 

The  plan,  however,  after  all,  was  not  found  to  be  practi- 
cally successful.  There  were  so  many  difficulties  and  in- 
cumbrances  to  interfere  with  the  easy  and  convenient 
management  of  it  that  it  never  was  carried  into  extensive 
operation.  One  good,  however,  results  from  the  experi- 
ment :  it  affords  an  excellent  illustration  to  aid  the  young 
student  to  understand  the  nature  and  the  operation  of 
pressures,  and  the  modes  of  measuring  them,  and  Law- 
rence made  very  good  use  of  it  for  this  purpose  in  hie 
conversation  with  John. 


THE   JOYOUS  LOCOMOTIVE. 


309 


CHAPTER  XXXIL 

CONCLUSION. 

THE  steamer  by  which  Lawrence  and  John  made  their 
passage  up  the  North  River  arrived  at  Albany  early  in 
the  morning.  From  Albany  they  were  to  continue  their 
journey  by  land.  Their  route  lay  to  the  eastward,  toward 
New  England.  The  scenery  along  the  road  was  very  pic- 
turesque and  beautiful,  and  the  locomotive,  as  if  equally 
proud  of  the  large  company  of  neatly-dressed  passengers 
under  his  charge,  filling  the  long  train  of  cars  which  he 


THE  LOCOMOTIVE. 


310  CONCLUSION. 

had  to  draw,  and  of  the  beauty  of  the  country  through 
which  he  had  to  take  them,  ran  whistling  along  his  way 
as  if  his  heart  was  filled  with  gladness  and  joy,  now 
winding  around  the  point  of  a  rocky  hill,  now  running 
with  redoubled  speed  down  a  long  incline,  but  always 
bringing,  at  every  moment,  new  scenes  of  fertility  and 
beauty  into  view — smiling  valleys,  pretty  towns,  and  for- 
est-covered hills. 

John  was  much  interested,  as  they  went  on,  in  observing 
all  the  streams  flowing  through  the  valleys  which  they 
could  overlook  from  the  windows  of  the  car,  and  he  saw 
many  examples  of  such  streams  pursuing  a  very  meander- 
ing course  through  level  meadow-lands,  which  had  every 
appearance  of  having  been  formed  by  the  filling  up  of  an- 
cient lakes  or  ponds.  The  case  in  which  this  effect  was 
manifested  on  the  grandest  scale  was  that  of  the  windings 
of  the  Connecticut  River  at  the  foot  of  Mount  Holyoke. 
The  travelers  stopped  over  one  train  expressly  to  obtain 
a  good  view  of  this  valley,  which  object  they  attained  by 
going  partly  up  Mount  Holyoke.  They  did  not  have  time 
to  go  to  the  top. 

When  at  length  they  took  their  places  in  the  train  again 
to  resume  their  journey,  John  amused  himself  with  reading 
for  a  time,  and  then  finally  shut  his  book  and  said  he  was 
very  tired. 

"  I  suppose  you  did  not  sleep  very  well  last  night  on 
board  the  steam-boat,"  said  Lawrence ;  "  and,  besides,  we 
have  had  a  somewhat  fatiguing  time  of  it  to-day." 

So  Lawrence  proposed  that  John  should  place  himself  in 
a  comfortable  position  and  see  if  he  could  not  go  to  sleep. 
John  said  he  was  sure  he  could  not  go  to  sleep,  for  he  was 
not  sleepy. 

"  You  can  put  yourself  in  a  comfortable  position,  at  any 
rate,"  said  Lawrence," and  then  I  will  tell  you  a  story." 


LEBON.  3U 

John  said  that  that  was  exactly  what  he  should  like. 
So  he  placed  his  feet  upon  the  valise,  and  leaned  his  head 
upon  Lawrence's  shoulder,  and  Lawrence  began  : 

"  I'll  tell  you  the  story,"  said  he, "  of  the  man  who  first 
discovered  the  mode  of  lighting  by  gas.  His  name  was 
Lebon.  He  was  a  Frenchman,  and  an  engineer  by  profes- 
sion. He  was  in  the  government  employ,  being  engaged 
in  superintending  certain  public  works  and  manufactures. 
But,  besides  his  regular  business,  he  was  greatly  interested 
in  making  investigations  and  experiments." 

"  That  was  a  good  thing,"  said  John. 

"  Yes,"  replied  Lawrence, "  if  it  was  not  carried  too  far. 
He  was  charged  with  neglecting  his  regular  duties  in  or- 
der to  gain  time  to  make  his  experiments.  I  do  not  know 
whether  the  charge  was  just  or  riot,  but  I  advise  you,  if 
you  make  any  experiments  this  winter,  not  to  let  them  in- 
terfere with  your  regular  studies." 

John  did  not  answer.  The  truth  was,  he  was  beginning 
to  feel  a  little  sleepy. 

"  The  first  experiment  that  he  made  in  relation  to  gas," 
continued  Lawrence, "  was  something  like  our  plan  of  dis- 
tilling gas  in  a  pipe,  only  he  used  a  glass  bottle  instead  of 
a  pipe.  He  observed,  in  watching  the  fire,  that  the  flame 
sometimes  seemed  to  nicker  in  the  air  at  a  little  distance 
from  the  wood,  and  he  conceived  the  idea  of  separating  it 
entirely.  So  he  filled  a  glass  bottle  with  sawdust,  and 
fitted  some  kind  of  a  tube  into  the  mouth  of  it,  and  then 
put  the  bottle  into  the  fire  among  the  burning  coals." 

"  But,  Lawrence,"  said  John,  partially  arousing  himself, 
"  the  bottle  would  break." 

"  Yes,"  said  Lawrence, "  if  he  put  it  in  suddenly  it  would 
break,  but  you  can  heat  glass  very  hot  if  you  heat  it  very 
gradually,  and  Lebon,  no  doubt,  took  all  necessary  precau- 
tions. His  experiment  succeeded  very  well,  Then  he  tried 


312  CONCLUSION. 

it  on  a  larger  scale ;  "but  the  gas,  as  he  first  formed  it,  had 
many  impurities  combined  with  it  which  gave  it  a  bad 
smell.  He  had  a  great  deal  of  trouble  in  contriving  modes 
of  freeing  it  from  these  impurities,  but  he  succeeded  toler- 
ably well  at  last. 

"  He  had,  however,  a  great  many  difficulties  to  contend 
with.  His  salary  was  very  small,  and  the  condition  of  the 
government  at  that  time  in  France  was  so  unsettled,  that 
what  was  due  him  was  very  slowly  and  irregularly  paid. 
All  his  friends  and  acquaintances  laughed  at  him,  too,  as  a 
visionary  schemer. 

"  He,  however,  persevered,  and  at  length  succeeded  in 
getting  his  invention  so  far  perfected  that  he  constructed 
an  apparatus  sufficient  for  lighting  a  house  which  he  hired 
for  the  purpose,  and  then  he  advertised  his  plan  and  opened 
his  house  once  a  week  or  so  to  the  public,  on  the  payment 
of  three  francs  admission.  Do  you  remember  how  much 
three  francs  is,  of  our  money  ?" 

John  did  not  answer. 

"I  verily  believe  the  boy  is  asleep,"  said  Lawrence,  speak- 
ing to  himself;  "so  much  the  better.  Sleep  will  do  him 
more  good  than  any  story." 

So  Lawrence  did  not  disturb  him,  but  let  him  sleep  on, 
and  John  did  not  wake  until  he  so  nearly  reached  home 
that  he  did  not  ask  for  the  rest  of  the  story.  I  will,  how- 
ever, add  that  poor  Lebon  did  not  live  to  see  the  final  suc- 
cess of  his  invention.  In  the  midst  of  his  active  efforts  to 
induce  the  government  to  make  arrangements  for  giving 
his  new  mode  of  illumination  a  fair  trial  on  a  proper  scale, 
he  was  found  one  morning  murdered  in  a  public  park  in 
Paris,  or,  rather,  in  a  wood  which  has  since  become  a  pub- 
lic park  of  great  celebrity,  but  which  was  in  those  days  des- 
olate and  lonely,  and  the  resort  of  thieves  and  robbers.  It 
was  supposed  that,  in  crossing  this  ground  on  his  way  to 


ARRIVAL    AT   HOME.  313 

his  home,  he  was  waylaid  and  killed  by  the  highwaymen 
that  infested  the  place  at  night,  on  account  of  its  very  dark- 
ness and  obscurity. 

He  lost  his  life  thus  for  want  of  the  safeguard  in  lone- 
some places  which  simple  illumination  affords — a  safeguard 
which  in  those  days  could  not  be  provided,  but  which, 
through  his  discoveries,  was  soon  to  be  introduced  into  all 
the  principal  cities  of  the  world. 

Indeed,  in  every  sense  of  the  word,  one  of  the  greatest 
means  of  protection  for  the  community  against  the  preva- 
lence and  the  consequences  of  vice  and  crime  is  Light. 

Early  in  the  evening  Lawrence  and  John  arrived  safely 
at  their  respective  homes. 

O 


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